If you are on the twillight side in phase 3 and a wipe is called and you leave through the portal, and then vanish to avoid the repair bill, it seems to bug the encounter with Halion will not despawn until you walk into a fire and die. Actually, if you vanish you can use engineering to shock a healer in to life and let him resurrect the raid and then you can engage where you left off. This is clearly an exploit
Shadowmeld and the other vanish-like stuff may work, aswell, but we haven't tried yet.