Half of us didn't notice until we had wiped out the rest of them and wondered why we were still in combat.
He was sitting up on a platform, evading, then after a minute of staring at him wondering what was going on, he jumped down and started smacking me in the face.
Then for another minute or so, he was immune to all damage. 4 people died shortly after.
After that, he started taking damage leaving myself, a tank, a rogue and a healer to bring him down from 20 mil or something.
Our druid was streaming at the time (Warning: Profanity, terrible quality and bad puns) http://www.twitch.tv/adriael/b/461801816
Jump to 48 minutes.
We have no idea how or why this happened, and armory hasn't reflected our kill yet.