There's an unkillable (even with AOE) mantis in the floor of the house that contains the PVP portal. You only see one mantis there now, which if you kill it (as I did in this video) you'll note that I'm still being attacked. Even using AOE abilities I can't seem to touch that bugger.

Please jump to the 2m55s mark to see that (I captured this bug ALSO on the same video where I was recording a frame rate issue.)