It is extremely common for answers on this site to quote either "There are no hidden rules", "Spells do only what they say they do", or often both (example), almost always regarding 5th Edition Dungeons and Dragons. There are also a few questions asking where these principles come from (example and example) – the answers typically refer to Tweets by Jeremy Crawford (a designer of 5e).
But the quotes seem somewhat trivial to me. What could rules mean except what they say? And if rules are secret from both the GM and the players, they don't functionally exist for a tabletop game. So I wondered whether there were, or are, other famous RPGs that somehow broke these principles in a way I hadn't considered.
Things That I Don't Consider to be Answers
I know that Paranoia specifically tells its players not to read the rules. But I don't think this counts, mainly because the secrecy is mostly a joke (discussion here).
There are games like Mao where the rules are deliberately secret from some players, but again, not from all of them; also, figuring out the rules is the whole point of the game. Although I can easily imagine an RPG with a similar aim, I don't think it would really count as an answer.
All multiplayer RPGs have social rules, which are usually left implicit. "The aim of the game is for everyone to have fun", "Don't harrass or bully the other players or GM", etc. While important, these rules are never the target of "hidden rules" comments, and so aren't answers here.
Errata don't count, because the intention is that all players and GMs read them (and that future editions include them in the main rules).
Publishing new books with extra spells, classes, items, etc. does not really count, because those are additions to the rules rather than changes.
Do/did any major RPGs have "hidden rules" or "spells that do things other than what they say", in contrast to the common statements about D&D 5e?
If unsure what counts as a hidden rule, consider:
- Do I need to know this to effectively play my part of the game? A player doesn't normally need to know the exact statistics of monsters, but does need to know how the details of how their spells work.
- Is the information hidden from me (or at least is very difficult to find), in a way that seems unreasonable? If the information is in the GMs' Guide with a notice saying, "This is for GMs only", and the GM is supposed to not tell me if I ask, then it probably counts as hidden (unless, like Paranoia, players are supposed to read it, but lie and claim they haven't). But if the hidden information is clearly something that a player is supposed to work out by logic or experimentation ("What happens if I cast Fireball on this rock with a picture of a fire on it?", "How do we discover the murderer?"), that doesn't count because it is reasonable to hide it.