1) No Mercy Rooftop Finale: The 4 of us were using the closet next to the gun-cage, and Louis (not a bot) decides it is a good time to throw a molotov... dang thing hits the door-jamb (or something) and sets us all on fire in the middle of a rushing hoard.
2)Blood Harvest Train level: This was the large warehouse the the team approaches after exiting the forest (again, no bots) and Bill gets attacked and taken down by a Hunter in the corner of the warehouse. Louis jumps in (I am certain that this person was new to L4D) blasts the living daylights out of the Hunter (and Bill) and then realises a horde is approaching through the loading dock doors. He whips out a pipe bomb and deftly hurls it in the general direction of the approaching horde.... except the pipe bomb hits the wall above the door. Louis did not notice and quickly turns to aid Bill. The pipe-bomb hits the wall and bounces back toward Louis and Bill and comes to rest about 3' from them.... and explodes.
I watched all this from the rafters as I attempted to provide cover-fire all the while laughing my arse off... and my mic was on
Disclaimer: I am just as guilty as anyone else for doing inane stuff (I've set the team on fire), but this is all in fun and not meant to be demeaning...it was just plain hilarious.