On Immortal WW Barb, Torment 12, killed Lair Boss Butcher just fine. Restarted fight to kill him again, got him down to 1/4 HP like normal, then instantly died. Checked Hall of Fallen Heroes, shows Slain by Unknown Killer, which is the same thing that showed on my Paladin when he was insta killed by a bug in Infernal Hordes.