The only way I know, is that the match finished, sometimes that can happen if there is lag. The match finishes, but the message never gets to your client. It also happens the opposite way around, a player makes the final move, assumes the match is over and leaves. The command never gets to fibs, and you actually have a saved game.
If your opponent has the ability to record matches, maybe they have a match record showing the final move? If there is a dispute over a league match in this way, a match record is traditionally the way to solve it.