While you're playing multiplayer you get a red text that says Out of Synch error and at the bottom right you have red #'s next to the human players name. Typically 2 or more players will have the same value and one is different which theoretically allows that one player to rejoin--i've never had that work and it requires saving and then reloading from that save. Rarely (for me) that allows the game to continue, more often it means the game is done and we need to start a new one.