I've logged in/out of a dungeon before, but that was messed up thing earlier in week (first dungeons) - Duty Finder dropped me in that Copper Bell Mine with a party-in-progess, I had to make my way to boss chanber (where they were) and got a lil lost when I used the short-cut, boss area locked me out. So after realising it wasnt to let me in I reluctantly bailed and DF put me back in again, that time I got there in time
People doing that on purpose is just petty and rude, no need to ruin other peoples games.

Oh I'm a tank btw, new to it though so be kind if we meet online