Sometimes, apparently, the server doesn't register that you've logged off if you DC while zoning. (!) This happened to me last night, and now I'm unable to log in Shmi Shmi, even after trying all kinds of nonsense (resetting every part of my network path, my PC, reinstalling the game -- even sleeping 8 hours). Some part of their system (the lobby server?) believes I'm still logged in.
Sqeenix created a thread for this issue, then shut off their forums. Apparently there were over 7,000 replies to the thread before the forums went down. I'm expecting to play an alt for the rest of the weekend. Yay stress test -- it worked! (I mean that seriously - I'm super glad this issue was found before launch). :)
EDIT: as of 10:15 am pacific Sunday, the thread is open for posting again. It's super duper slow, and grew by 100 posts between my post and the thread refreshing! Yikes.