I let tanks die when I get healer aggro and they don't bother to pull off me after I run into range of the tank's AoE abilities to let them just stand in place to pull things off. Even worse is when the mobs they're targeting die and they just run on ahead to the next group of mobs. I like to think I'm teaching them to keep things from chewing the healer's face, but it's honestly just amusing to watch the tank pull a group of four, die instantly, and have the mobs reset while the dps and I kill whatever is attacking me.

Not sure why, but it's happened several times in the last week or so. I always thought it seemed like common sense to keep the healer from taking damage, since it makes their job that much harder.