One situation which I now know triggers this, and still does so on r4394, is when a joining player hits the "play now" button repeatedly (maybe more than once is sufficient). Zhis player will then join, but the game doesn't resume. If the server admin tries to resume it manually, it does so, but just for one frame, then pauses again. Further attempts to resume the game result in the same behaviour, so the game is no longer playable.