...you only quit playing after the smell of your own body odour is too much to bear.
...your girl/boyfriend stands in front of your TV completely naked, promising to do that thing with her/his pinky that you always wanted to try, and you tell them "Move! I'm in the middle of a boss battle!"
...you started playing on a Monday and before you know it its... Monday.
...the carpal tunnel and tendonitis don't stop you from finishing Fallout 3
(That last one is from my own experience. Sad really, but I finished the damn game)