This sorta qualifies as a Harrass the GM story...
I was running a horror game ( Now if any of you have been in a horror game, you know that mood and atmosphere are key to a good game..Keep this in mind) that involved several ghosts, one of them with the ability to possess people. The game was coming to a climatic part of the story where a ghost had possesed a maid who then tried to kill some of the PC's. During the fight, one of the players managed to decapitated a poor possesed maid with a lucky shot with an axe. The dialog at the table went something like this..
GM: The maid's lifeless body crumples to the floor. Blood is everywhere.
PC 1 : I whack the body with the axe again just to make sure.
GM: Ok.... While you raise your axe to deliver another blow, the head of the maid begins to speak...You find it odd that she now is speaking perfect french! (Pause for dramatic effect)
PC 1: I find it odd that a decapitated head is speaking at all...
The whole table, including myself erupted in laughter. It was impossible to recreate any sort of atmosphere after that...
Just goes to show you that even when the GM thinks he/she is in total control...