I once had a tank in Cathedral of Eternal Night that wrote a lot of insults in the chat and refused to kill the WQ boss (this was early broken shore, we wanted that rep). There was no provocation from what I could tell, he was just really angry for no reason. In the end he left when the rest of us decided to just kill the (WQ)boss without him.
5 min later, we got a new "tank" I say it like that because he was in fury spec, and we rushed through the rest of the dungeon without a single wipe.(heroic)
Most tanks are OK, but since the only really hard part about it (on lower difficulties) is the anxiety that comes with the responsibility, it is no surprise that you get the occasional egomaniac.