One thing about the challenge that confuses me.
Why do they replay the point and not award the point as it's a wrong call? (Although, that Fed one there was a bit stupid)
Well that would be unfair, because both players stop playing once a call is made unknowing of if its right/wrong. You see when a ball goes long for example, the player recieving doesn't hit it back because of the call, it would be unfair to award a point against him when he could have easily carried on playing but stopped because of the call.