Back when my wife and I first got together, in the days when The Burning Crusade had yet to happen, World of Warcraft had a very predictable maintenance window of Tuesday mornings. This did not work out well for me, due in no small part to the fact that at the time, I was working in the evenings. Tuesdays, thus, were a period of time during which I basically could not play at all. It was exceedingly annoying.
Of course, I think we've all run into some maintenance times that have just not worked for us, for whatever reason. Maybe the maintenance starts exactly when you get home from work, maybe it always seems to be an emergency session when you really want to play. Heck, maybe you're playing something in Early Access that always is up during times when you're not playing a live game and down when you want to do something. So what's the most inconvenient MMO maintenance cycle you've encountered?