As soon as we get in the tank says he's never done it before and do we think he should drop the group. We all try to be reassuring, since everyone's had to start somewhere and his gear was certainly good enough for a heroic. Then the first pull began and we quickly learned that he wasn't kidding about having never done it before and that the answer he was actually hoping for was that someone else would tank, because he clearly had no idea what he was doing, period. Furthermore, he was ridiculously hyper-critical of the healer (who didn't seem to be having any significant issues healing the group) and would go on long tirades blaming the healer for not having healed him at all when the healer was clearly dropping heals on everyone in the group including him. Drop to 90% on a trash pull because you ran up the ramp out of LoS? Yell at the healer. Take damage from one of the ghostly skulls and not get healed to full before pulling the giant skeletons? Rant at the healer. Frankly, I have no idea why the guy took it.
Finally, after the last pull before Devourer of Souls, where he deliberately went and chain pulled three of the Spectral Warders despite the healer having said OOM and then blamed the healer for letting him die when he had a 75% healing reduction debuff and the healer was out of mana, I'd basically had enough.
Now, I could have just ranted at him, or dropped group. But that felt like abandoning the healer, who had been working very hard in mostly iLevel 200 blues and had in fact kept the entire group up through the tank's not knowing to move Brohnjam away from the soul fragment or to collapse to the center so that he stood outside and Icy Touch spammed while the group stood in the middle and DPS'd. So instead, I decided to do what I suspected the DK wanted me to do back at the beginning of the instance. In the middle of yet another paragraph of text about how the pally hadn't healed him at all I put on my tank set, switched specs, and said in party "Okay, if you don't trust his heals you don't have to tank it anymore. Pulling."
It was a remarkably smooth kill for a group that basically four manned it (the former tank I'd usurped didn't have time to switch specs or get on any DPS gear, which is as I'd intended) and I received an angry tell from the former tank calling me various expletives and profanities, which only made the "Wow, thanks for stepping in" comments in part all the sweeter. Yes, it absolutely was a jerk move on my part, but reading the healer's "Yay, thank you" afterward made it worth it to me. However, it completely ruined my data collection for my passive ArP test, so I'm going to have to go queue up for another random as DPS now. I guess nothing is perfect.
So now I ask you, gentle readers: have you ever just said "enough of mister rock star" and done something like this? Or do you think I was totally unjustified?