The tank is not a tank.

"Did I mention the tank is a tank?"

"Sold!"

But seriously, for those who may have been chain running Heroics to gear up their alts, nothing is more annoying than the 5-10% or so of "tanks" that are not tanks. I'm not talking about bad or new tanks, I'm talking about the really crappy DPS that queue as Tank for a shorter wait and then hope someone either: drops group so they can requeue as DPS, but be in a group of 3-4 and so get in sooner anyway, or takes over tanking with their offspec. Do not do this. Wait until you can kick them, and do so. Taking over for them and not kicking them is exactly what they want, and dropping out of the group (if you're not the healer) is a really close second. Don't give them what they want and they'll stop doing it.