Now that we're nigh unto finished with the dungeon crawl portion of our program, I'm curious what the best trap or trick you've ever used or encountered in a dungeon has been. Ideally we're looking for something triggerable that the PCs could avoid, or a situation that the PCs have to Encyclopedia Brown their way through in order to get into a specific part of the dungeon or to a special treasure. No fair just referencing White Plume Mountain either.
In my case a two come to mind: back in college Viv ran a session where the PCs, in order to get into a certain part of the dungeon, had to get through a very deep aquatic area. The room leading into it was cramped and was described as having stale air, but the torches set in sconces in the wall continued to burn merrily. We players had to figure out that there were magical air spheres around the torches, that those were portable, and that we could remove them, stick our heads into them and head on into the briny. That frustrated us no end for some time until the answer became blindingly obvious.
The other was a trap layout in a Kobold lair - and I don't recall now how much of this was in the module - where the kobolds had mastered work in paper mache. The PCs would, upon heading up the slope of the kobold's cave, likely trigger a trap that would a) remove the supports under a stretch of paper mache camouflaged to look like stone at the base of the slope and b) release a huge boulder to roll down the slope at them. The PCs would very likely flee in a suitably Indiana Jones esque fashion only to the find that once they were at full velocity the floor under them was collapsible, dropping them into a pit. Then the boulder would fall into the pit over them and would itself be revealed as paper mache, which is good. It also contained some white faced hornets nests, which was bad. The trap was the perfect blend of terrifying, embarrassing and likely non-fatal designed to make sure the players would grow to hate Kobolds forever with the white hot fury of a thousand suns.
What've you got?