It's hard to think of many horror stories on my end, and I've done many PuGs.
Maybe it all just blends in with the learning process of how to carry random groups.
The only remotely recent one I can think of was the last time I tried to do the alcove method for the waves in H HoR. I had done it successfully a couple times on my warrior tank, but this was the first time doing it on my death knight tank (the waves at least, I got thrown into a PuG doing the escape sequence some time earlier with him). Needless to say, all it takes is ONE player being slow in getting into that corner to cause problems (in this case, an arms warrior who used bladestorm every time it was off CD). We wiped repeatedly and I was starting to lose my cool (a VERY rare occurrence), before we simply called it. I actually stopped tanking for a week or two at that point, so great was my annoyance.
.... and then, in a strange twist of fate, I heard about the hallway method. Gave tanking another shot, only to get tossed back into H HoR as the first random I've tanked in a long time. The hallway method didn't just work, it had to be the smoothest run of the place I had ever done up to that point (and they've only gotten better since). It wasn't too long ago that I got tossed into H HoR on my DK tank, and I hadn't returned there (as a tank) since that one fateful day. It was a group that had gone through no less than two other tanks (both DKs no less) and had wiped several times already. I could only say one thing...
Yeah, wasn't looking forward to it. Well, you won't solve any problems if you don't even try. So I gave the hallway method a go, and it was a great run that garnered applause from the group. Most people can't believe how much easier the hallway method is compared to the alcove, but I think this group really appreciated it. Wiping several times with other DK tanks, only to have another one come in and give them (or help them, it's all the same) a wipe-free clear? It just feels good.
I guess that was a horror-redemption story of sorts, but I think the first bit qualifies for horror. Getting over it was quite nice though.
He who isn't afraid to stand by his ideals...
Isn't afraid to stand alone.