A spriggan was stuck in a corner. It could not leave the corner to follow us, it damaged us wherever we went, dropping aoe on it from the floor above didn't damage it and we couldn't target it. We tried leaving the group (separately) and reforming, but it just ended the instance anyway. We couldn't leave the instance and come back as we were in combat. This was somewhere in arc three with all threads intact. Here is a screenshot, best I can do, of the problem:
Favourite NPC: Wine-For-All
Mostly PC-EU , with a lonely little guy on NA.