Hm, disengage as hunter in worst places ever (e.g. Pit of Saron, forgot the *Pit* part for a second there and weee...);
I also sometimes cast moonkin form instead of flight form on my druid when I jump off high places when semi-afk and realize that a bit too late :-\
Also, keeping barkskin next to entangling roots on action bar. Yay, tanking Onyxia, everything is going ok, almost end of phase 1- ah, crap, misclick -> humanoid form -> Onyxia makes a druid-kebab from me in 0.1 sec...
Can't remember my ''biggest fail ever'' right now but ''small yet consistent'' fails happen to me on a daily basis. Meh :
Actually, one of my ''bigger'' fails was getting a flying mount and dual spec on an old paladin alt, lvl 60, which i never played again and ended up deleting him. :<