Onyxia can get some really bad PuGs. You can watch people stand in the middle while all the others run to the side. You sit there and wonder, "Are they scratching their head right now wondering where everyone ran off to?"

Worst PuG raid element I run into is ToC's first boss. He does a charge that everyone must dodge or he gets an uber dps buff. The raid I was in a week ago warned everyone that they must get out of the way (dodge left or right) and that's all they have to do. Well we wiped 4 times because some jackass thought they were smarter than the system or just were plain clueless. Had a mage iceblock, felt like reaching through the internet and slapping them.