Video games in general are great ways to spend your downtime. Unless you're between games, there's always a new battle, a new place to see, or something new to find. Add in having all your friends around, and playing in large worlds and MMOs tend to amplify the point. There's a reason we make jokes about "just one more level" or "just let me finish this quest" among MMO players - just about everyone's been sucked in to that point at one time or another.
Of course with that comes the problem that will generally at least once bite every MMO player; getting so sucked in that you're late somewhere to do something. This morning we thought we'd ask what the single-worst case of losing track of time you've experienced or heard about is?