In a competitive tourney, an ump said foul by mistake...at that point, it was a dead ball, even though several of us saw the ball land fair. The ump admitted his mistake, and told the other team he goofed, but he could not change it, as we had stopped playing the ball. If a HR, it could have been changed, but not a live ball...it turned dead when it was called. A fair ball called foul, one of the minority of calls that can't be overturned/corrected. If, in real-time, the ump had said, 'Foul -- no, FAIR!', I think the play could have & should have continued.