I don't mean I pulled aggro. I mean that when I queued, I forgot that I had tank selected alongside DPS. I do this in five man heroics I'm running for justice points as well. When I find myself selected to tank the dungeon (often only noticing after I get in and no one else is the tank) I usually shrug and put on my tank set and do it. It's not the group's fault I keep forgetting to uncheck that box, after all. And there's a bit of an up side. The other day my wife and I were talking in game and I said "I think I'm going to ride my blue dragonhawk" which surprised her, because I am not a mount collector. "Wait, you have a blue dragonhawk?" Well, yes I do, and I can thank forgetting to uncheck that tanking box for it.
I'm under the impression that I'm fairly rare in this regard. I don't know how true that is, because I've really only talked to a few people about it, and some of them don't play hybrids, so there is no other box for them to check. I'm sure all the warlocks I know would select tank if they could, for instance. But at least some folks seem to do this from time to time.
Being an opinionated cuss, I have some thoughts on this whole phenomenon I'd like to share.