This one, which I wrote for the “Tomb of Horrors” super-adventure. (Spoilers!)
An open archway reveals a massive cubic chamber with a 3-foot-high white stone altar at its center. The characters know that this chamber contains a hidden exit leading to their goal, but there’s no sign of secret doors in the floor, ceiling, or walls. On the altar sits a skull whose eyes and teeth are gleaming jewels. No sign of danger can be seen in the room — but if the characters specifically search the floor along the walls of the room or along the bottom of the shrine, they notice a thin seam.
Any tests of the floor reveal it to be solid, because it is — a solid stone slab set at the top of a 90-foot-deep pit. The white stone altar is actually the top of a 93-foot-high stone column, around which the floor is set.
If the altar is touched, the magic holding the floor in place is disrupted, whereupon the floor shatters and falls. Any characters in the room drop 90 feet, taking damage as they hit the ground within several tons of tumbling rubble. A minute later, a false stone-slab ceiling at the top of the pit also falls, shattering to drop several more tons of loose rubble to the bottom of the pit and on top of the injured characters scrambling around in it — or those savvy enough to have triggered the falling floor remotely, and who are in the process of smugly descending, convinced that they’ve outsmarted the DM.
The central pillar is hollow, and the exit from the pit can eventually be located through a cleft halfway up its side. Assuming there’s anyone left to find it.