On the summit, in the north section of the map, right before the mobs start walking down the stairs where they have a straight path to the crystal, orcs have a possibility of getting stuck. The problem is resolved by aggroing them away, but if you're playing on pure strategy, it's super annoying, because you can't aggro them.
The orc will just walk around in circles, unable to make it around the corner.
So far every time they have eventually died, but it sometimes takes minutes on end of having only one more mob to kill.