Tobold asks a question: How do you Carry a Tank?
It's pretty rare, but I've seen a group carry a tank before.
I define "carrying" as when a character performs significantly below normal, to such an extent that it requires other characters to perform significantly above normal in order for the group to be successful.
I was once put into a Dungeon Finder group with three DPS and a bear tank, all from the same guild. The bear tank had a 150k health, so I thought all was good.
As we progressed through the instance I noticed two things. First, the DPS was very good, all pushing 10k. Second, the bear was taking a ton of damage. I was having to chain Divine Lights and pop cooldowns to keep him up. I found this amazingly odd, given that the tank came from the same guild as three top-notch DPS.
Mid-way through the run, I inspected the tank. The tank was in full PvP gear. So he had a lot of health, but was missing all the avoidance and mastery that would have significantly reduced his damage taken. He was also missing the talent Natural Reaction.
The tank had enough health so that he wasn't one-shot, but he took damage at a rate that required me to burn mana at a much higher rate than normal. However, the DPS performed at such a high level that they were able to end the fights before I ran out of mana, in addition to requiring the barest minimum of healing themselves.
It's fairly obvious that the group decided to have their druid respec and wear PvP gear, and let the DPS carry him through the instance, rather than face 45 minute queue times. I think the Natural Reaction thing was just a mistake, as the druid spec looked good otherwise.
The point is that because the tank performed at such a low level, the DPS had to perform at a much higher level so that the run was successful. The performance of the DPS effectively carried the performance of the tank in that run.
The moral of the story is that very high DPS forgives a lot of sins.