Most commonly this happens as a healer or tank. Mostly healer though.
As a healer, I always try to heal everyone. Generally even if they are being a douche. Same thing with tanking. I don't make myself look bad just to prove a point, but sometimes it is funny to do so.
I have a WoW friend called Moparman. He is a mage. He has tried a bit of tanking, but he is completely beyond noob. He didn't know to tank in frost presence. He doesnt know rotations or even spec to be honest.
He does things sometimes just to give you shit. Once while I was tanking (we were on vent) he said, "Damn this tank can't tank for shit". To which I replied with a click from group ;P
Just yesterday, we were running Heroic Occulus. On his mage, he was standing in near melee range and getting hit by a blizzard. I politely said "Get out of the fucking blizzard, you arent melee!" He said "Yeah, but you will heal me." Of course I told him that he was going to die next time.
He has obviously never played a healer. Someone who hasnt played a healer or tank, can't always fully grasp the importance of that role and even concepts as building aggro or mana conservation. They just go balls out and hope that it all end up ok.
Quit being fucking wankers. You should try every role in the game so that you (as the Onyxia Wipe guy would say) "Know what the fuck to do". Learn other roles and it will make you a much better and aware player. You will also cease to look like such a dickface. The end.