The WHM part of your post confused me for a second. At first I thought you were saying never to use medica 2. Lol.

Know what another bad habit is? BRDs or even sometimes BLMs going balls to the wall on a boss before a tank even has the chance to use shield lob/tomahawk. Then the BRD/BLM will turn around and QQ that he has aggro for a few seconds. It actually happens fairly often, unless you're lucky and get a group of vet players.