Consider the meenlock, a fey creature of pure terror described in Volo's Guide to Monsters (page 170). As GM I am going to have my players encounter these creatures in our next session. They have been stalking the countryside, abducting villagers and turning them into more meenlocks. However, I have found what seems to be an inconsistency between their stats and the prose describing their behaviour.
They 'reproduce' by telepathically tormenting an incapacitated victim.
Up to four meenlocks can telepathically torment one incapacitated creature... If the creature is susceptible [not immune to the frightened condition] and remains incapacitated for 1 hour, the creature must make a Wisdom saving throw, taking 10 (3d6) psychic damage on a failed save, or half as much damage on a successful one... The process can be repeated. A humanoid that drops to 0 hit points as a result of this damage instantly transforms into a meenlock.
Meenlocks have two strategies for this according to Volo. One strategy is "to crawl out of their tunnels to torment sleeping prey." This is not the strategy I have questions about, as a sleeping creature typically has full health. Their other strategy raises some questions.
Meenlocks like to paralyze creatures with their claws, drag them back to their hidden den, beat them unconscious, and telepathically torture them over a period of hours.
To beat someone unconscious requires dropping them to zero hit points. Their paralysis only lasts for one minute, so unconsciousness is the only reliable way that a meenlock can keep someone incapacitated for a whole hour. So meenlocks would frequently torment creatures that are already on 0 HP.
But here we have an issue. The telepathic torment is guaranteed to deal at least 1 point of damage (unless you are resistant to psychic damage, in which case you have a 0.5% chance of avoiding damage if you pass the save). But because the target is already at 0 HP it will invariably take damage and be left at 0 HP by the telepathic torment (if not killed outright), which means that this telepathic torment will invariably take only one hour regardless of saving throws or damage rolls, or so it seems. This is in contradiction to Volo's description that this torment lasts several hours. It is also means that a captured creature has no capacity to resist this effect despite it notionally offering a Wisdom saving throw.
An alternative interpretation is that it is not possible to 'drop to 0 hit points' when you are already at 0 hit points. This appears to be how death ward is interpreted. However, if we take this interpretation, then it becomes impossible for the meenlocks to perform their strategy of beating someone unconscious and then tormenting them. The meenlocks would have to wait for the target to recover hit points before they can successfully torment them, but if the target recovers hit points they stop being incapacitated so cannot be tormented.
What happens when meenlocks use their telepathic torment on a victim they have beaten unconscious?
Does the damage inflicted on the unconscious victim invariably turn them into a meenlock after the first hour? In this case Volo made a mistake when he wrote that the process lasts for "a period of hours". It also means that the rules for damage and saving throws for telepathic torment are only useful when the meenlocks target a healthy, sleeping victim. From my own reading of the rules this appears to be the most fitting interpretation. I would have liked the process to be longer, for the sake of dramatic tension and chances of resistance and rescue, but I can make do.
Is telepathic torment unable to transform a creature already at 0 hit points? While a strict rules-as-written argument could be made for this, I feel this interpretation is directly contrary to how meenlocks are meant to work.
Is it some other outcome? Maybe I missed something. Maybe there is a plausible and consistent way for meenlocks to beat a creature unconscious and have to spend several hours (rather than just one) tormenting them before the transformation occurs. But without homebrewing the telepathic torment mechanics I can't see how that could happen.