For all of the pugfail I've seen lately (mainly driven by Spring Break, I believe), it's also nice to know that when some bad things happen, some people can step up to the plate and deliver.
Take a Gundrak run a week or so ago. The initial pull started off okay, but then for some unknown reason the Rogue ran ahead of the group and into the snake area and managed to aggro all of the rest of the trash not on the tank. The Rogue, of course, proceeded to die very quickly. The tank was able to get everything else to aggro on him and between he, the Mage and I we were able to take care of the rest of the trash.
The tank then turned on the body of the Rogue and pointed.
"You!" he said. "If you EVER do that again, we're kicking you out."
Properly chastised, the Rogue was rezzed and slunk back into the back of the pack, behaving himself for the rest of the run.
On another run -Halls of Stone, this time- we lost the healer to the second trash mob. One minute he was there, the next we're all saying "Heals? Where'd you go?" Turns out another mob had aggroed on him and he was trying to avoid it. In the process, he ended up aggroing a couple more trash mobs, and he had this pile of Dark Iron dwarves trailing him as he was running away.
Straight. Into. Us.
My, what an ass-kicking that was.
After we all released and ran back into the instance, the healer apologized and for the rest of the run he stuck his tail-end right next to the tank's.
In both cases, we could have easily kicked out the person who screwed up, but after calling him out and giving him another chance, they turned out to be a valuable member of the pug.
No, you can't assume this will happen all the time (or even part of the time, for that matter), but I thought them notable enough that I wanted to mention that yes, it can actually happen.