I had to stop playing quickly a few weeks ago (baby woke crying). I was literally a few seconds into a DF Garuda HM run and I logged off, assuming that would remove me from the instance. I was astounded 30 mins later to log back in and discover I was still in the fight, the party trying to win with just 1 healer! It is crazy that actively logging off (as opposed to sudden disconnect) does not remove you from the group / instance. I expect many people make the same perfectly reasonable assumption that it does.