I just played a game where I turned into The Amazing Reno. At the start of one of my turns, the hero power auto-cast Corrupting Mist with 3(?) minions on the board. At the start of my next turn, no minions actually died. The hero power then happened to cast Corrupting Mist again (likely a coincidence). Since I was now paying attention, I noticed that no minions actually had debuffs placed on them from Corrupting Mist.
Perhaps a bug with this being cast in the start of the turn, where the debuff is supposed to trigger?
Anyway, definitely should have cost me the game, but I ended up luckily dodging a Ragnaros shot (who lived for 3 more turns than he should have due to this bug).