Hmm there have been a lot so here is the most recent two. Both are from left 4 dead 2. They are a little long but it's worth it I promise.
On the third part of no mercy where you have to hold out at the elevator for a while I somehow managed to be the last one alive when the elevator arrived. I was at the far end of the hallway in that room with the surgery tables. 4 infected (it was versus) were waiting for me and I was low on ammo. The corpses of my teammates were spread through the hallway.
Now, did you ever see the lobby shootout in the first matrix movie? Because that's exactly what I did

I burst out of the room spraying bullets everywhere. When the clip was empty I picked up the weapon from the nearest corpse and kept spraying, never reloading. That way I made it to the elevator, killing 4 infected along the way using 5 different guns in 20 seconds. The server went nuts.
Another time, funnily enough in the exact same hallway, me and 3 friends were holding out for the elevator in the same room. Just when the elevator arrived I spotted an M60 (the supermachinegun) in the corner and picked it up. Now normally at this point you stay in the room a little longer until the hordes stop coming. But I felt like doing something crazy so again I burst out of the room running around and spraying like crazy, emptying the gun by the time I got to the elevator. This took my friends so much by surprise they were still in the surgery room when I was already in the elevator.
The audio chat went something like this:
elevator: ding!
friend: ok just stay in here a sec lo-
me: *bursts out* YAAAAAAAAAAAAAAAAAAAAAAAAAHAHAHAHA BWAAAAAAAAAAAAHAHAHAHAHAYAYAYA
-10 seconds of total silence-
friends (still in the surgery room): DUDE! what the... DUDE!!!
me (in elevator): you coming?
friends: umm yeah sure eh... holy cow what was THAT?
me: what? that's how you use the m60 no? There wasn't even that many zombies.
friends: you're kidding right? you got all the specials!
me: what? no I didn't?
friend: yeah you did! You even got the hunter in midair!
me: lolwut?
I honestly didn't see, not even the little red message at the right of the screen when you kill a special. "What hunter?" has been our inside joke for weeks now ^^