I think there is a rare in game damage bug that occured for me in Onyxia the other day. I started noticing 6k mind flay ticks, and then a 14k mind blast as displayed by the default scrolling text and recount as well. This was totally surprising to me, and I instantly checked my buffs for anything unusual. I had all the "normal" raid buffs and nothing extra was showing.
I can only conclude that this is a rare damage bug similar to the one that can occur from the vehicle item level modifyer carrying over after a buggy flame leviathan encounter.