Yesterday I was in the forest instance solo-ing mobs when somebody joined me and zoned in. They proceeded to grab aggro on a few mobs and killed one of them in what I assume was an effort to level up. After they leveled up, they quit the instance, and the mobs which had aggro on them became stuck in the spot where they were attacking.

I proceeded to clear the zone of mobs only to still see the mobs that were stuck still appeared on my minimap. When I went back to attack them, they had no collision detection at all and I couldn't take them down.

This was my first instance attempt, so I'm not sure if anything was supposed to happen once all the mobs had been cleared in the zone. If something 'was' supposed to happen, I couldn't achieve that goal because the last mobs had no collision.