You may sometimes see a green clock appear on your screen in the middle of the match, which may indicate your opponent has lost his data connection. Your opponent has 30 seconds to re-establish their connection, and if they don't, you win the game through a technical win.
On some occasions, the green clock may still appear on your screen and continue (or even finish) the countdown, but you'll still see your opponent making moves.
This can happen when the connection is remarkably slow and the game takes a few seconds to sync. That means your opponent was already back online, but the game took a few more seconds to sync, this results in the game continuing while the error message is still displaying.