The most baffling thing happened to me during a random dungeon run. The hunter was misdirecting to me. Me: the resto druid running around in cow form casting pretty green lights and efflorescence and moonfires.
Once? Must be an accident. I roll my eyes at the pug and continue on with a little laugh.
Twice? ...that's no accident. Eyebrow raised and growing irritated, I had to say something.
Yes. I had 6k more health than the prot pally, therefore the resto druid healer (who was not, incidentally, running around in melee at the time) must be the real tank and should be misdirected. This was followed by the resto druid casting /facepalm.
Lesson: the highest hp does not mean tank. Look before you misdirect, please ;)