Sorry to hear about your dog dude. That's always harsh.
This is piddly shit compared to what Zee and The Underpants Man posted, but it did annoy the shit out of me so here it is: I was playing through The Parish campaign on L4D2, which is the campaign I have the most trouble with out of all of them (as expected since it is the final campaign). And I was rocking it pretty hard despite the Director being dickish enough to put a Wandering Witch right in the middle of the Gauntlet Crescendo at the decontamination trailer, where it was impossible to avoid her and really difficult to kill her. And it was surprisingly easy, otherwise, there were minimal Witches (all in locations where it was easy to sneak around them) and only one Tank.
Then we got to the impound lot, full of alarmed cars. I was hoping against hope to get the Violence in Silence achievement, since the bots and I had encountered so little trouble so far. But, the Director went full on dick. Only a couple of Special Infected spawned while we were inside, which is unusual and got my hopes up even higher... and then I found the cars had been set up so there was no way through without walking on top of one
And even though I've been told the alarm shouldn't go off from standing on a car with an alarm, the moment I jumped up to go around it it sounded. So despite getting through only having to fight one horde of zombies, and coming out pretty well unscathed (thanks to me swinging a machete like crazy and running circles around my near defenseless bots to keep them safe), no achievement for me