Several of the mercs can get the special ability To the Death, which causes the merc to attack random enemies until either merc is dead or all enemies are dead. Now, to start with, this ability is just simply too good: paired other abilities that provide immunities, like Commanding Shout or King Mukla’s Primal Power make this ability just oneshot everything. Trash, any boss, all dead in one move, splat. Not necessarily bug, just wildly overpowered. At least you know what to do if you can’t beat some heroic boss any other way.
What happens when you send unstoppable force against immovable object? What happens if you use To the Death with immune character against an enemy that is also immune? Such as Ahune the Frostlord. Or your attack power was temporarily lowered to 0, causing the merc to bash on To the Death quite infinitely. Frankly, I don’t know, because there’s no easy way to produce the perfect conditions, and I don’t feel like wrecking my game just to confirm a bug. But my guess is on the infinite loop and alt-F4 to exit game. If not, good job anticipating it.
Another, very similar combo I did witness. It resulted in tens of attacks and could very well have ended up in infinite loop as well: Mr. Smite reacted to taking damage by attacking my char and healing a bit. Unfortunately, my char had Radiant Breastplate, which caused Mr. Smite to attack again. And again. And again. He healed just enough to not die until the Radiant Breastplate broke a SPUD bomb that finally got him (thus exactly 20 attacks later), while his damage was too low to do any harm to my character either as Thrall kept healing me per every incoming attack. Without the SPUD there, they’d probably still be bashing each other.
So, consider this a “review infinite loop chances” of all kinds bug/warning.