I don't know if that works for someone else too, but I have the impression that if I tell a group it is the first time tanking on a dungeon, some smart dps will have the fun of targetting wrong mobs just for the lol of taking away aggro and then bitching I'm a bad tank.
First time tanking, it is not first time playing the game. I just found a good number of dps who does that on purpose.
Is that a common issue or I am just unlucky? :)