People suicide for no reason.
My last one on Maethori, everyone suicided because we only had one tank. Despite two people saying that I could tank (I hate doing it, but I can). Or when you only have one cleric with revive.. Or when you have some stupid mage spamming "rez plz" when you only have one cleric, and we're at a boss.
I think the worst has to be when you're grouped with a bunch of people who don't have a clue what they're doing, and just charge in. And get everyone else killed. Or simply don't want to go out of the "norm" for a KQ. Or when Clerics decide they're better tankers, even if we do have a fighter who can tank.
Argh! KQ's are so frustrating sometimes.