Mostly it's because the servers go down and you dc and try to log back in.
But I got something wierd happening... I died when I was exping, but when I died i fell into a tree so I couldnt get rezzed, i tried to go to server 2, same problem, server3, same problem so i said, meh i'm going back to o1 and try to click on my body as soon as i log in. But when i closed my game and tried to log back in, it said account in use. I pressed Ok a milion times and nothing happened.
I did ipconfig /release /renew even flushdns
But now I still get account in use error... WTFBBQ?