As a healer, I never forget my lesson about not putting a regen on a tank mid-pull.

I was doing The Stone Vigil for the first time and tank was pulling some trash mobs. He was quite fragile at that time and I was a quite inexperienced white mage. So I had the idea of putting a regen afterwards to help keeping him alive (since he was reaching 40% HP pretty easily). On that pull, the mobs decided that the tank was lame and gone after my head. I died before tank could pull them off me and the party wiped afterwards.