About a year ago, me and some other people at work started playing multiplayer Civ4 together during our lunch breaks. Then the length of our lunch breaks started to increase. Then we started leaving the game open in the background while we were working. Then we started bugging each other over IM whenever it was someone's turn and they hadn't moved yet.
Fortunately we were able to self-moderate and stop entirely before we got yelled at. It was pretty bad week for productivity.