It couldn't have been poison and fire if he had unevolved Graboids in play.
And in my experience, this almost exclusively happens when playing the Fire-based halfbloods -- I strongly suspect that it is a result of the interaction between the Firebolt animation (which takes a while) and the new "the instant you hit 0 HP you lose" rule to prevent Antimatter healup. The Firebolt is cast, the game realizes that you hit 0 HP, and the You Lose screen pops up before the animation ever reaches the second frame.
