One of my first experiences DMing was a 2e game that I was playing with some school buddies. Most of them had played before and were serious about it, but of course we had that _one guy_ who thought it would be _hilarious_ to try to cause as much trouble as he could.
So, his plan is to break in to the local castle and try to sneak into the princesses' room...I let him get all the way up to the 3rd floor of the castle before I set some guards loose on him:
Player: "I run down the hallway...is there a window there?"
Me: "Yes."
Player: "I run for the window, and jump out into the moat!"
Me: "There is no moat..."