"Bad Vibrations" got my vote simply because this is a real unavoidable nightmare and can happen to any of us at any time. Many of the others were really the result of either bad judgement, bad planning, bad management decisions, or just plain rookie DBA inexperience. In any event they are all "preventable" in my opinion with a little planning, coordination, and forethought. But having multiple drives go down on you at one time I have experienced myself and I can tell you first hand, this is not only a unavoidable "nightmare", it is a long one too!