Tonight I was with a full party. doing a run on Haukke Manor, and we came to Manor Sentry. I believe that at half health, this mob is supposed to fly and open the doors to the boss chamber. But in our run, the Sentry just stayed in place and became invincible... preventing us from continuing on with the dungeon. We proceeded to run away and see if it would reset... the health on it did, but it was still stuck in the same place we left it, invincible and ugly as ever.

Let me know if I can offer any other details.