I got two.
The first and most recent was on "No Mercy" when a friend opened a door, behind which was .... well ... the witch, he starled her thus causing the usual oh shit jesus christ f*** f*** response while back pedalling and turning her into swiss cheese.
the second is more elaborate but oh so funny.
Let me set the scene.
blood harvest, last level, the ledge leading into the cornfield (has the sign "keep your hands up in the air" etc) a friend is sat next to me, his entire health bar is filled only by pain pills, mine is at half health.
"There is no way in hell I am going down there" my friend says, surveying the field.
A Roar rips through the eerie quiet.
I turn around.
WALL TO WALL MOTHER F***ING ZOMBIES!!!!
screaming the usual array of profanity that l4d causes, we back pedalled off the ledge, throwing molotovs to create a wall of fire.
Unfortunatly zombies don't instant die on contact with fire.
Thus as we ran back watching flaming corpses run at us through the field, we screamed, we shot.
We got crippled.
lying on the ground, getting mobbed by flaming zombies.
as we realised we would die, we just burst out laughing.
Shortly before i died, i heard the telltale bleep of a pipe bomb, unfortunatly I also saw the red light, the idiot had thrown it at me.
Ka-Boom!.
no more me.