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.
If you stay to do the job, do it right
I always put on my tanking gear and try and tank. Now, I have pretty good gear for both tanking and DPS - frankly, I outgear heroic five mans and I'm well ahead of the curve for LFR (I'm just doing these for valor in most cases) so I could definitely tank a LFD group in my DPS gear/spec if I wanted to, and with a good healer LFR is probably doable that way. But it seems wrong to me to do that. For one thing, I know we're at the end of expansion phase of the game, but we won't be there forever, and people get serious bad habits that will get them killed when the content gets hard again. You can't do much to stop that, but you don't have to contribute to it, either.
If you're not willing to do the job, then leave - don't waste people's time
I know it sucks to end up losing your queue position because you accepted a dungeon and/or raid finder group and then saw that you were queued to heal when you're a DPS paladin as your main spec. But I've seen lots of LFR groups fall apart because one of the healers was DPSing poorly instead of healing. Same goes for people who queue as tanks, then do so in DPS gear and fury spec, or who flat out just plain refuse to do anything. I get that those people are looking to get kicked from the group, but sometimes we can't kick you. Just suck it up, own your mistake and go do something else with your time.
Don't sleepwalk your way through
I'll illustrate what I mean by this by a story of a group I did recently. It was a Scarlet Monastery group, and I only realized at the end that I was queued to tank it when I got the tank bag (no mount, though) - I just assumed the tank was terrible, and I guess in a way I was right since I was he. No one complained and we cleared it fine (overgeared as we all were) but it was an example of me not paying enough attention. Had I known I was the tank, I may have asked if they minded if I went as fury, because everyone in the group was SoO geared - but it's way better to ask and be aware than to just stumble through the dungeon not paying attention because you're arguing about whether or not dragons should be able to have kids. (For the record, I think they should.)
So that's my story of accidental tanking. Have you accidentally tanked or healed any dungeons recently?