The funniest game I ever played in was a one shot session of All Flesh Must Be Eaten. The scenario was called Blizzard of the Century. It had been ran at Origins a few years and a friend contacted the guys running it and got a copy. He then made some modifications and ran it for us.
The scenario was campy goodness. Set in the late 70's, we were a group of college geeks trying out this fantastic new game called Dungeons and Dragons. Our characters were snowed into an apartment complex. A demon was unleashed and began to create zombies from our neighbors.
It was a blast!