Army of Two.
Last level on the hardest difficulty, you start and you're supposed to get into this lobby through the only 2 FRONT doors. no possiblity of flanking until you get your huge bullet magnety ass inside. There are 2 strategies for this: get your partner in first, get him to aggro and sneak in, or you get the aggro first and get HIM to sneak in. First way : He dies near the door, i pull him outside, revive him, and try the 2nd way. I get inside, get behind a wall start getting aggro, order him to get in. He runs in, gets behind the shittiest cover EVER, so I decide to try and spam the "move here" order so that he gets behind proper cover, the fucker won't listen. Meanwhile, I'm not shooting so aggro goes on him and he dies. I try to run to him and revive but also get shot by the millions of infinite spawning cunts in the room.
Checkpoint respawn.
2nd try, we get in again. He finally gets some good cover so I tell him to take aggro. I sneak as best I can, but apparently the enemies in this lobby havent heard about the aggro system. They all turn towards me and shoot me. Partner runs in to revive me. He drags me for several seconds to some very shitty cover. Starts to heal me but gets shot! so he stops and shoots at the enemies (hitting nothing though). when everyone in the room decides to stop shooting he just stands there. When does he decide its a good time to revive his teammate? When everyone STARTS shooting again. He gets shot and stops again. Rinse, repeat until I bleed out.
Checkpoint Respawn.
3rd try, I decide that he doesnt WANT to live so I tell myself "Fuck it, I'll let him bleed for a while while I clear the room cuz that fucker is a moron anyway!" I quickly realized that that won't work since he's going to bleed out like a little pussy and fail the mission. So I have to drag him through it some more. Same drill as last time He gets aggro, enemies still seem to ignore hime completely though and shoot me instead. I go down, he runs up to me, drags me over to Cover? NOPE! right in the middle of the lobby behind NOTHING as cover. Due to some miracle of god though, the revive succeeds... Yeah, go figure. However when I get to the receptionist desk, my AI dumbass companion decided he liked the view from the centre of the lobby, probably due to the lack of cover that wasnt obstructing that view. So at this point one of the heavily armored guys spawn (the ones who only take damage from behind.) and I decide that I'll need to take aggro from him otherwise my companion will die out in the open. So I begin to shoot the fucker. And he pretty much has time to walk casually up to me and take me out, while my companion is shooting at nothing particularly enemy-like. I die, he dies.
Checkpoint Respawn.
It took about 20 tries to get past that fucking lobby. but When I finally did it I was ecstatic!
I also had a Left 4 Dead Witch story. Basically a group with 1 ****, who was on a ledge above the area, that decided to shoot a witch, who was in the bottom part of the area, while I was about get on the ladder to get on the ledge. The Witch was about 4-5 meters away from me and decided to hit me instead of the moron who shot her. (Think this was in the sewers area of one of the levels. Right before you hit the switch to lower the gate and the mob of zombies spawn. Witch was pretty much underneath the switch, and the ladder to get up there was only a few feet away from her.)