Not terribly funny, but I enjoyed it:
The characters had been asked to investigate several disappearances near an abandoned tower. After being attacked by giant rats, they cautiously made their way into the entrance. The rogue decides to take his time and check for traps, and the player is surprised there aren't any (there weren't).
They proceed into the hallway, where the rogue's player decides not to check for traps. So, of course, no one is prepared when the floor gives way beneath them.
After climbing out of the hole, they proceed down the hall to a door. Opening it, they see a small library with several skeletons in it. The skellies attack, and the players respond. Unfortunately, they're all equipped with bladed weapons, meaning they keep missing!
So, one of the players gets a bright idea. He flips up one of the tables in the room and uses it to pin a skeleton to the wall. He then declares he's going to try and knock the monster to the floor (he succeeds).
Now that he has the skeleton on the floor, trapped beneath an overturned table, what does he do?
Player: "I start jumping up and down on the table as hard as I can!"