My friend was playing a Demon Hunter and was using some kind of caltrop/bomb ability when this happened. He had just leveled up and was trying the ability, but I think it had something to do with the bug. We had to continue on the map for another 10 minutes with those mobs following us before we were finally able to kill them. There were no magic effects around them or anything, so I know it wasn't an "intentional" invulerability and it isn't something that particular mob ever did again.
I wish I knew the name of the mob, but they were the burly looking guys like the ones that come out of the monsters that cling to the walls. I remember it having silverish looking plate armor on.
It isn't a major bug, but if we didn't run, we would have gotten killed and in a Hardcore game that might generate a little grief.