This bug was on testlive and now its still present on live (even after the hotfix).
Sometimes, when you kill a mini boss (represented by a white skull on your minimap), this skull is not removed and lingers on your minimap till you leave the instance.
When more then 1 skull bugs on your minimap, it becomes very annoying (esp. in higher waves).
This is how a “cleared” zone looks like on your minimap: