I remember playing a game once that was ridiculous for the sheer number of ways the adventure tried to mess with adventurers. Phasing behemoths unhindered by the deep sand the rest of the party was in. Ten or more gaze attacks per round.
But the encounter that really made my day was the foyer of the main dungeon. Once the party was inside the room, all the doors locked and we were attacked by…the floor. Yup, some crazy mage trapped the foyer so that the floor animated and attacked intruders. But the best part was when the ceiling animated as well and joined the fun, providing a flanking bonus for the floor!
So the next time your group of heroes enters a dungeon looking for things to fight, don’t forget to check the floor, walls, and ceiling.