Recently in a few dungeons I've been in the healers have used rescue to punish other players as payback for not playing how they want, I've seen healers run ahead and drag the tank into a new set of mobs, I've seen them purposely pull players into aoes and watch them die...
It has literally just happened to me as I didn't get hate off the healer quickly enough in a normal raid, (I was the ot, I don't know why the Mt lost hate then) yeah, I could have been better and I am trying to learn, but the pettiness of it (I was told that's what I get for not getting hate fast enough) really surprised me. I don't tank much, but have been trying to learn. I can see why people just don't bother.