Consistency between "yes and" with "no, because I'm the DM"

Question

Context: I enjoy running softer systems, with Numenera being my latest favorite. I discard rules left and right in favor of what feels right during the play. I love the "rule of cool" too, and I use it liberally.

One of my players is rather creative and often vividly imagines the outcome of their plans. How do I help them manage their expectations when these plans can not come to fruition for DM reasons?

It's easier to explain this with examples. Here's an example of me letting the player have fun with their creative solution:

• DM: The bad guy is monologuing safely behind a force field.
• Player: I use my cypher of bypassing force field and my superhuman speed pill I swallowed 3 hours ago (and they indeed have) to tackle the bad guy.
• DM: Give me a roll.
• Player: 20.
• DM: You succeed.

The rest of the players jump in to restrain the baddie, everyone celebrates and players have fun. I'll survive not having a bigger showdown here.

And here's example of me not letting the player to go forward with their plan (all the rolls are 20s in this example to double down on the problem -- I gave the player opportunity to roll, they critically succeeded, but I don't want them to):

• DM: The bad guy is leading you to his office.
• Player: I "accidentally" tap the bad guy on the back to make sure they're not a hologram.
• DM: They're not.
• Player: I'm walking last, and I put on my invisibility cloak.
• DM: Give a roll.
• Player: 20.
• DM: You succeed.
• Player: I sneak behind him and slice his throat while he talks (this seems like a murder-hobo response, but the character had a solid motivation behind this)
• DM: Give me a roll.
• Player: 20.
• DM (trying to save a recurring villain for a larger ark): You attempt to do so, but in the last moment the bad guy wiggles out and yells for guards. A fight ensues.

Villain activates a mysterious device and teleports away during a fight. Also, since this is a Numenera game, I give out my players some experience for the GM intrusion and out of some guilt.

How do I manage players expectations without discouraging them from finding creative solutions to problems presented during the game?

Answer 1

You are creating your own problem

As mentioned in Thomas' answer, rules are the way the game provides consistency and sets the expectation of the outcomes of the players' actions. If you arbitrarily dismiss them as you want, you are creating your own problem.

In particular, "because I am the GM and I want to" is a very poor justification, and will probably (rightly) frustrate the players, making them feel your decision was completely unfair. It is okay if you have a simulationist reason, or some actual reason, but then you may explain that reason to them, rather than just saying "because I am the GM". Honestly, it does not even need to be a great reason, but I would rather hear a bs explanation such as "the villain had a third eye in his back neck and saw what you were up to" than "because I wanted to".

I win button

Other than that, there is some great advice on this article by the angry GM (as usual, strong language, proceed with caution) which talks specifically about the "I win button", which is:

Eventually, a player is going to attempt an action that, if successful, will resolve all remaining conflicts or answer the dramatic question with certainty. Sometimes, this action won’t even require a die roll.

...

If you are running your game properly, and a player drives the ball off the tee and scores a slam dunk, you have to give them the two-point conversion. If you don’t speak sportsball, basically, if your players find a way to win the encounter in a single action, even one without a die roll, they won the encounter. If you can’t stomach that, you can’t stomach being a DM.

So, basically, if you have considered that the action was able to succeed (and I assume you did, because you let them roll for it), you should have rewarded the player for finding such an easy win.

"Aw, but I do not want my strong and important villain to die in such a way waah" - Then why, for everything that is sacred in this world, did your villain allow the player characters, fully equipped with their Cloak of Invisibility and weapons, to stay behind his back, without anyone guarding them properly? I am not familiar with Numenera, but if coup de grace exists (i.e., slicing the throat without any damage or HP check, instantly killing the enemy), then I do not see any reason the player should not have succeeded their attempt, and the only way I see to be consistent was to accept your villain messed up and let him die.

As a player, I have only had very bad experiences with GMs arbitrarily changing rules because they did not want their major villain to be beaten easily, and every time it happened, it was frustrating to me and to the other players. You mentioned the rule of cool in the beginning: what would be cool here would be to accept your players found a very good way of dismissing a threat in a very good opportunity where he presented himself vulnerable and reward them accordingly.

So, the only thing I can say about using "I am the GM and I want to" to save a villain from a very good and valid action from your players that would cause the dismissal of the villain is... don't do it.

I mean, even you know it was a bad call, otherwise you would not be giving them XP out of some guilt.

The attack succeeding doesn't mean the end

To be clear: The attack having succeeded does not mean the campaign, or even the story arc, is over. You could end the story arc and still the players would have all the other problems of the world to solve. But this villain dying doesn't even mean the story arc itself is over. Other answers have focused on this, so you can get ideas and explanations on them, but, as examples, you could have a smarter henchman or second-in-command take place, or this villain was just a puppet, and the master mind will just choose another puppet to keep doing their job.

Still, as a reward, you could make the plans of the organization fall behind on their schedule, or the second-in-command presents himself as an easier final fight than the previous one.

Maybe you don't even have a problem

I can only see two exceptions for the previous section: Either you are playing a system that deliberately states that the GM can do whatever he wants based on its own whim and nothing else, thus the players expect such arbitrary decisions and are more likely to accept them (and with a 400 page core rulebook I doubt Numenera is such as system) or you explicitly agreed on that in some kind of Session 0. If that is the case, then you do not have a problem at all. The expectation was, from the beginning, that any action is a lottery and outcome will be based on GM's will. If they signed up for that, they signed up for that.

Don't ask for meaningless rolls

As a final, general GM advice, unless your intention with the roll was that a good roll would result in the outcome you described, and a bad roll would result in a way worse outcome, I fail to see the motivation behind asking for a roll that seems meaningless. If even in a 20 the character would still fail to accomplish his action, why even roll? At the moment you asked for a roll, you implicitly infused the player's brain with the following expectation: "If you roll high enough, you will succeed". Then, he sees a 20, and he's like "OH YEAH DAMN YOU BAD VILLAIN". Then you proceed to describe how the attempt was futile.

You could, again, explicitly tell your players that sometimes you are going to ask for rolls even if the action is an automatic success or automatic failure, just for the thrill of it, or whatever reason. But this is not, in my experience, the usual expectation.

Talk to your players

If you have not done so yet, then you should be clear with your players about the expectations they should have. If they are okay with arbitrary decisions, then you don't have a problem. If they are okay with not knowing what to expect from their actions, you don't have a problem. If they are not okay, then you should consider following the advice in this answer.

Conclusion

To summarize

• Talk to your players. Be clear about what should be their expectations. Find out if you really have a problem.
• If you do, consider changing how you have been running the game, or finding a table that fits better for your style.
• "Because I am the GM and I want to" is a poor justification. Give a good in-game justification. Even a bad in-game justification is usually better than the "I want" justification.
• If you can't find a good in-game justification, then probably you should not take that decision. From the player's perspective, if you do that, that will not be "cool", or even fair. If your players found a good way of solving a problem that is easy and fast, good for them.

Answer 2

The rules are the primary tool a player has for aligning their expectations with realistic outcomes.

As you state in your introduction, you "discard rules left and right in favor of what feels right during the play." Because you have established that you are capricious in your rulings, you have taken away the primary tools a player has for managing their expectations concerning the consequences of their actions.

If I know my DM is going to apply the rules as written with a reasonable measure of consistency, I am equipped to think through the space of outcomes my actions might have - I am aware of all the possible rules that may apply, and what different outcomes might look like when I roll the dice. But if I know my DM is consistently inconsistent in their rulings, every situation becomes a two way lottery: I gamble on the whims of the DM, and the DM gambles on the players coming up with reasonable or unreasonable ideas.

What do you do with this?

I have two ideas. First, and this is probably less concrete than I would like, just be more consistent in how you rule things. Be more sparing with your rule-of-cool rulings. Rule-of-cool is best reserved for cleverness and ingenuity; and remember, clever and ingenious ideas are not outlandish and unrealistic ones.

The second idea I can offer would be to work with your table to codify some house rules. If there are any situations that come up with any regularity where you depart from the rules as written, write them down. Give yourself a list of house rules you can be consistent with, and re-equip your players with the tools for managing their own expectations. There is trouble with this solution, though. There exist both good and bad house rules. Some maintain balance, some can break balance. Your mileage may vary here depending how extensive such a list of rules become and what rules your table decides to make. The first suggestion is surely the most reliable solution to your problem.

Rules-lite is, of course, a valid play style.

To be clear, every table is going to be different with respect to their adherence and reliance on the rules as written. This is okay. Even at tables where rule of cool is the name of the game, the rules as written is still the primary tool the players have for managing expectations. But if this is your table, you must be aware: this makes the rules a less reliable tool for this purpose. Because this makes the rules a less reliable tool for managing expectations, you absolutely must make up the difference with communication. Consistent communication before, during, and after play about rulings is critical for maintaining fun at the table and managing expectations.

As a DM, this communication is going to be your primary tool for managing expectations. The more your players are inside your head, the better they will be able to manage their expectations for play. Talk through your rulings with them after the session is over. Explain your reasoning. If you do this consistently, their understanding of your DM style will make up the expectation deficit created by a more rule of cool play style.

Answer 3

Sometimes, when you win, you really lose

This is a common theme throughout literature and cinema, and it often produces some of the best content. To quote Robert Burns, "The best laid schemes o' mice an' men / Gang aft a-gley."

So what if the players kill the villain? What happens next? Who was the villain?

Obviously, the front-man for whatever villainous scheme is being concocted. But, who is the brains behind the operation? Clearly not the guy who turned his back on a fully armed party. By killing the dumb front-man who was probably going to monologue and spill his guts about the plan, now the party has no opportunity to learn of what is about to happen. The true villain behind the scenes continues the schemes unhindered by the party.

Perhaps killing the villain leaves a power vaccuum. The ensuing fight among underlings produces a horrific outcome. The fighting spills onto the streets. There is guerilla warfare, and no one is safe. Without the established leadership of the villain, the fighting is chaotic and difficult to control.

Basically, solving one problem causes more. So, let the party win. Take the break between sessions to contemplate repercussions of their actions. In the end, the player's actions may help lead to an even better story.

If your players know their plans can always succeed, but may lead to unforeseen consequences, they may be less willing to jump the gun and take action without really thinking through the consequences. Remember, they are acting in the moment. You have the entire time between sessions to decide what the consequences of a single session might be.

Answer 4

A success need not be the success they intended

The character successfully slips on the invisibility cloak without being noticed by their mark (but...)

They sneak up on their mark, and go to slice their throat.

At the last moment, the villain's hand comes up at just the wrong moment, and bumps the would-be assassin's hand. Because they rolled well, the villain's reaction is to look around, confused, but fails to realise that what they felt was an invisible assassin's hand.

Meanwhile, the villain was already being watched (by some means) by someone who suspected them of villainy, and they recognise what just happened. Due to such skill in the assassination attempt (which was foiled by bad luck, but in a way that didn't expose the attempt), this other person decides to approach the party after the encounter and offer new information or some sort of boon, so long as the party agrees to continue countering the villain.

The villain survives, remaining unaware of the party's intentions, thanks to a bit of skill (as reflected in the rolls).

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This logic applies across situations...

A character tries to jump a chasm that they can't possibly jump? A good roll sees them recognise the futility of the jump at the last moment and grab a stray bit of well-rooted shrub, saving them from great pain.

A character tries to switch out a weighted treasure for a similarly-weighted substitute, not recognising that it's magically-protected and the switch can't work? A good roll sees them react to the faintest magical glimmer given off by the treasure and stops just millimetres from touching the treasure.

A character tries to deceive an NPC into thinking they're the NPC's closest friend, and there's no way the NPC would be deceived so easily? On a good roll, perhaps the NPC believes the character to have been hired by their friend for a laugh and gives away that this is what they think ("Did Jane put you up to this? Where is that trickster?" Looks around for their friend).

The benefit from a good roll could, in some situations, be of a meta nature. Perhaps it gives experience, for example, or can reroll one bad roll within a certain period of time.

Answer 5

Let your players know the stakes and potential outcomes of their decisions.

There is a lot of information going on in the story that the players just don't have unless they ask or you tell them. A character, proficient with the dagger, might reasonably be able to judge the difficulty of slicing an enemies' throat. The player, however, won't know this unless you tell them. You can be reactive about sharing information and only tell them if they ask (and possible succeed on a roll). The problem with this is that it can slow down the game with your players asking hundreds of questions to make sure their limited perception of the situation aligns with your design of the situation.

Alternatively you can be proactive and tell your players (or have them roll to learn) the relevant information regarding a proposed course of action.

In the second example you mentioned you could communicate to the player "This person seems alert and physically able, you won't be able to kill them with a sneak attack but you might be able to injure them". If the player only desires killing the enemy in a single blow then they know their strategy will not succeed and so will not attempt it. If they are content with injuring their foe they can make that calculated decision.

Unless it consciously done to support the story you should be honest with your players about chances of success, the results of success, and the consequences for failure of a particular choice of action.

If you turn a success into a failure, either by just saying it is a failure, or by inventing a way to punish that course of action, you are discouraging creative play. If you communicate that a course of action cannot succeed, or that it's success will be limited then you will encourage creative play because creative ideas will only ever have benefits.

Answer 6

Other answers are all good, but missing one key pragmatic element:

Don't be afraid to call a temporary recess.

Basically, be perfectly open with the players by looking surprised and saying "… I did not actually expect that. Ten minute break while I figure out what the heck this means." Take that time to sit back and think through things, figure out your alternatives, and just generally regain your mental balance.

Also, don't be afraid to ask the relevant players to step off to one side and discuss things with them. The essence of the "Rule of Cool" is that a good story for everyone should trump mechanistic rules. At the very least you should do this if you're going to have to nix the attempt, so that the player has a chance to absorb it, but better by far is to see if they have any thoughts on creative solutions.

I won't go into a "war story" here, but based on personal experience as an accessory (not the instigator) of breaking one of the best GMs I know:

1. If you really, really screwed up and truly cannot think of another way out of it, you can apologize to the players and explain that you shouldn't have even allowed the roll, for whatever reason. This is still a really poor choice, but if you own it and you don't do it more than once in a blue moon, players will forgive you. But try to save this for "no, really, neither I nor the player involved can come up with anything that doesn't really ruin the game for everyone" situations.
2. If you've done the background world-building, then when the players throw you for a loop, you can roll with it because all you have to do is think "well, what would the logical consequences of that be?" and go from there.
3. If the problem is that the player did something crazy that would turn into an "I win" button, then clearly there must be a reason why it doesn't generally work; let the player have the win this time and then help you come up with an explanation for why it doesn't work in the general case. Sometimes this can actually lead to some really interesting consequences that creative players can make use of in less game-breaking ways later.
4. Generally, the best answer is to let it happen and turn it into a plot hook. Great, the character just slit the BBG's throat… only now they're a wanted murderer with dozens of witnesses to the act. Or maybe just one very irate non-witness who can afford to hire a mage to scry what happened. It doesn't exactly take much to guess that "captive vanishes" followed by "three seconds later BBG has a new smile" might draw a really big target on said character's back, when they otherwise might not have gained that attention. This is arguably just an expansion of point #2.
5. If you really want to prevent something from happening by fiat, but in exchange let the player benefit, then you need to negotiate this with the group up front. I would point to systems like Fate, with its use of "fate points" to tokenize "the more I insist as a GM, the more you benefit as a player." But do take note that in Fate the expectation is that there is a limit on how high the "bidding" can go, and a player always wins ties. So at the end of the day, if a player has enough points to spare and cares enough, they can keep the success even if it causes issues, but they've also "had their Moment of Awesome" (for the time being) and others will get to shine more for a while.

Answer 7

If for whatever reason you cannot allow a player to succeed at an action, do not let them roll.

In the best case, avoid that they can even do that action, but if that failed, the very least you owe your players is that you make it transparent that they failed due to higher powers (the GM), not because of any lack of skill, hidden abilities of the villain or some other in-game reason.

This, of course, assumes that in Session Zero you clarified with your players that they are ok with that level of GM intervention for the sake of the story.

If they are not, or you didn't clarify, I strongly advise to roll with things next time. Do not take honestly-won success from the players without their explicit agreement. It is much easier for a GM to come up with a new villain than to find new players.

Answer 8

Remember hit points are a thing

The way that characters are meant to defeat their enemies is to drop their hit points to zero.

In your second example, your player is trying to bypass that whole system by saying "okay, I slit his throat and now he's dead".

The way I respond to requests like that is I say:

Well, you can make an attack on him, but it's not going to be an instakill attack, it'll just do whatever damage your attack usually does when you make a surprise attack. If you get a critical hit, it'll do critical-hit damage.

If you don't say things like this, you're going to get situations in which everyone says: "I'm not making a conventional attack, I'm just going to slit his throat" because that's way more effective.

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As to your first example, you might want to consider that being "tackled" doesn't automatically mean that someone is helpless. Maybe "tackled" is an asset that characters can claim on their roll.

Answer 9

The question you should ask yourself at the time (and the players in the discussion about expectations in the game) is: is this the kind of game we're playing?

So, character A turns their back on an opponent. Character B becomes invisible and executes an insta-kill move on character A. Character A is dead and gone forever.

Would the players want to play that game if character A were their PC? Didn't think so. I don't know whether Numenera has a fundamental asymmetry in the system which means the rule the PC used cannot be used by NPCs who want to kill them. But if not, if that rule doesn't really distinguish between PCs and NPCs, then "well, the rules say it's possible and my character would want to do it" just signifies that the rules don't suit what your players want out of the game.

If this wasn't even done by the rules, then it signifies that your rules improvisation doesn't suit the style you're trying to run, which is fairly easy to fix: just stop improvising rules that make it too easy to win like this. To quote Dickens, "If the law supposes that ... the law is a ass -- a idiot".

"Rule of cool" means, rule of cool. Cool and "winning very easily" are not actually synonymous, much as it might seem that way to the character. The character is not aware that their purpose is not to deal with the villain, it is to entertain the group of players.

There are several "correct" responses to rules that don't suit what the group wants from the game. One is to play a different game, but that tends to lead to a costly and disappointing process of elimination. One response is for the GM to take on a responsibility to maintain the mutually-agreed style of the game (and often to define that style without explicit agreement beyond "we want to play your game"), even if this means playing a bit loose with the rules. It seems that's what you want to do, and sometimes this means curbing the players' enthusiasm a bit. So go ahead and do it, provided the players agree that's what you should be doing. Alternatively, if they want to play the insta-kill game and you're willing to run the insta-kill game, then do that instead. But the only way that can be the style of the game is if you actually do kill them whenever they make similar mistakes. Which, let's face it, averages more than once per session for most groups. I mean, seriously: what precautions do they normally take against invisible enemies coming at them day and night?

In general, though, you should not be saving an NPC just because you think they're cool and you've put work into them. Realistically a player who pulls a move like that is expecting one of two things to happen (they may or may not have an opinion which it is they want to happen):

1. The NPC goes down, they high-five each other and collect the XP.
2. They discover the next obstacle in the game, that is to say it turns out not to be so simple and they're forced to overcome additional challenges before (1) happens.

Case (2) is deeply unsatisfying for the players if your response reeks of "nah, I'm not ready for that to happen yet, so my plot-relevant NPCs are just damage-proof until I say otherwise". What you need to convey is some combination of, "ok, so that level of ruthless insta-kill is so far from the action style this game was intended to center on, that's it's not even available to you as a viable option" and "ok, that's a nice idea, but this game will be more fun actually if I don't have be smarter than all the players combined, just to get a line of evil monologue out"!

So, carry on but do better at explaining why their plan didn't work. You are allowed to take a moment to think about it at the table, so, what it is about the game world that means everyone (including NPCs) don't go around constantly murder-hoboing? You're looking for something that doesn't make you feel guilty in the way a straight-up "he wriggles free" did (or in general: "it doesn't work, because, um, well it just went wrong"). Then:

• Out of character: can you suggest to the players that their PCs might show restraint for the same reasons that everyone else in the world does? Let them do the thinking, how this actually plays out in their PC's mind. Then, they can still be creative but need not start from, "we win by killing the plot" or, even worse, "we win by exposing the GM's lack of foresight".
• In character: can you inflict reasonable consequences on the PCs when they do this kind of thing and it fails? Like, if it works on a natural 20 (and your plot is ruined) but the other 19 times it doesn't work (and the otherwise-unarmed PCs are taken down by the very-much armed guards and go straight to the torture chamber, so their day is pretty much ruined), then the PCs aren't going to consider it such a great bet. Just as in "real life" they might consider that assassinating every problem is tempting but probably not a good long-term strategy. So, they can still be creative but within a better set of constraints than, "stuff PCs try basically always works, and stuff NPCs try basically doesn't because PCs veto it".
• In character but "cheating": can you just decide that regardless of the rules, a grade-A villain probably would not be as vulnerable to random back-stabbing as that, or else they'd be dead long before the PCs even got to them. Therefore, the villain is smarter than you (the GM). Therefore, if they're turning their back even on a probbly-unarmed crew of enemies, then they must have arranged some protection that you were too dumb (or too pushed for time) to think to write into your game notes. Improvise that protection, and if the PCs still fight their way past it then they earned their kill. So, they must still be creative but you're going to demand more than one clever insight to win.
• Earlier in session planning: can you create scenarios where knifing the villain in the back is (in the medium-term if not the short-term) not a win for the players? Maybe they need something out of the villain other than his corpse, and killing him will just mean they need to get that something somewhere else. The fight happens later, once the villain has taken some step (I dunno, invading Canada or something) where killing them starts to look like it really will improve the situation.

This isn't just relevant to PC vs NPC lethality. Some games have a similar issue with the old sand-in-the-face trick. When the first player ever thought of it, and (I wildly speculate) the GM improvised a system which basically says "yeah, that worked, you have a massive advantage in this fight", everyone cheered. Then, when every PC and goblin in that game realised they're better off carrying a bag of sand in their off-hand than a shield, it got really boring really quickly and the house-rule got changed next session. Eventually games got invented where you literally cannot throw sand in someone's face during a combat other than by using a printed rule which covers that interaction. It's pretty obvious from your preamble that you are not interested in such games, so I will say no more about them ;-)

Sometimes players want off-the-wall stuff to work, but generally they don't actually want every "creative" idea to Just Work, even if they're the one trying it. Creative and quick thinking is not the same as devastating combat prowess, which is among the reasons Robin Williams was never a martial arts champion. Unless you're playing strictly RAW and the particular game rules leave you no discretion, part of the GM's job is to exercise some judgement to reward entertaining creativity, without over-rewarding it. The reward for thinking of a clever, cool way to backstab the plot is that something interesting happens that would not have happened if the player hadn't been clever and cool. It doesn't need to be that the plot dies and the PCs go home happy: anything which makes the players go home happy will do.

Short answer: if everything they try works just because it's creative and you didn't anticpate it, then they get to describe a lot of cool stunts but there's no actual challenge in the game, not even to their creativity. That might be what they want (in which case let them kill the damn NPC), but it probably isn't (so improvise back at them in order to make them work harder).

Answer 10

When I used to DM a lot I took the rule of cool a little too far and always got confused why my bosses just got stepped on by the party. Didn't help I had bad memory and my players had GREAT memory; they remembered whenever I gave them something game breaking, and made me regret it.

DMing is game design. It's the nuts and bolts of what makes everything "work" and you have to balance encounters and still make the story interesting and give your players agency. Giving players certain items or access to things is basically like putting a dozen aces into a deck and trying to play Texas Hold'em: it's going to get weird fast.

I started really consciously thinking of each session in a "box" and the ins and outs or whatever, and not lavishing my party with really fancy shit. If they HAVE to have their wild powerful items or whatever, you just have to get even more creative. Maybe big villain dude wasn't a hologram but like convinced the player he was someone else somehow, maybe he's does get knifed but oh literally immortal what now huh?

Last thing: like everyone else said: talk to your players. I ran a Star Wars campaign and was really disappointed with the outcome of last session; a character that should have gotten away was captured by the party. I talked with my crew, and we re-ran it. Instead of rolling bad and getting knocked out, he force pushed the whole party to the other end of the ship and sliced off one my player character's arms, and bailed out an escape pod. Everyone agreed it was more satisfying, even the dude who got his arms cut off!

Answer 11

“No, because I'm the DM” should definitely not be engaged after a die-roll, with a previously defined game mechanic involved, in which the player scored a critical success. Any one of those alone would rule out that being a good idea. This particular case happens to be a triple-whammy.

When can a DM legitimately say "no"? In my opinion, mostly when adjudicating reasonableness of stuff not formally covered in the game rules. Or communicating game-rules that clearly bar certain actions. Example "no"s: No, your standard PC cannot long-jump 50 feet. No, there is no pegasus for sale at the market today. No, your character doesn't speak the right language to communicate with the alien.

Separately, if you're going to say "maybe", that is, call for a die-roll, the result of that die roll had usually better matter. Personally, as a player, I can tell when DMs are calling for fake/meaningless die rolls and it stinks and makes me want to leave that game.

If you're improvising a die roll for some novel situation, then I'd recommend you get real clear in your head exactly what the different ramifications are of success/failure before the die is cast. Even better: Say it out loud to your players. This (a) commits you to the objective outcome even if you personally don't like it later, and (b) lets the players judge the risk/reward and back off if they don't like the prospects. I find that it intensifies the drama and tension in my games to know that we're playing for keeps in that way.

Possibly as novice DMs we've all made the mistake of having a player make a die-roll, on a gut level feeling "boy, I sure hope they do/don't make this one", having it go the other way, and then thinking "crap, what do I do now?" ... end-result: the die-roll didn't matter. We should be on the lookout and sensitive to those moments. Make a decision and own it out loud! Either say "no, that won't work" (and own up to your judgement), or say, "here's what happens if you succeed/fail" (and commit to that before the die-roll). The dice are there to drive the universe of the game once the probabilities have been defined. Don't cross your fingers and hope that the dice will take away your responsibility to make a decision about the good and bad possibilities.

But if there are defined rules for an action (like standard attacks), then you really shouldn't be ad-hoc changing or nerfing them on the fly. If you didn't want those mechanics, then don't false-advertise and tell players you're running a particular game system when you really aren't. (I've actually seen a guy at a convention pat himself on the back for that. "I advertise that I run D&D , but I really don't, I just list my games that way to get more players." Foul!)

But the coup de grace is to take away a critical success that a player just rolled; that's just going to hurt too much on the player's side. If criticals are a thing in your game, then obviously players are going to cheer and celebrate (or gasp with horror) when one comes up. Follow-through and give them that memorable, maybe legendary, victory. Taking that away just hurts me to read about it.

In short, any one of the elements listed in that anecdote, (a) changing a core defined system mechanic, (b) doing it after a roll, (c) doing it on a critical success, would have been sufficient to spotlight this as a bad DM call. All three together is just a perfect storm of bad faith.