Tried to run Hullbreaker Isle Hard Mode and ran into an issue on the first boss. One of our DPS ran in before the rest of the group entered and started the sequence. However, after the DPS backed out before the arena was sealed, everything in the arena disappeared, and didn't come back, rendering the instance uncompleteable.
Here's a screenshot of what happened, including arena trigger and reset times:
http://i.imgur.com/aqJ6VhD.png