If you accidentally press the 'windows' key (which is very close to the alt key) in full screen mode, you switch to desktop. This then causes the game to crash.
If you do this while in the middle of a leave quest, when you go back into the game you will find it 'cancelled'. You then have to spend an additional leave quota to re-attempt the quest.
This is a sad penalty for when the game unintentionally crashes on you :(