From Friend at Court:
17. Prompt calls eliminate two chance option. A player shall make all calls promptly. A call shall be made either before the player’s return shot has gone out of play or before an opponent has had an opportunity to play the return shot. Prompt calls will quickly eliminate the “two chances to win the point” option that some players practice. To illustrate, a player is advancing to the net for an easy put away and sees a ball from an adjoining court rolling toward the court. The player continues to advance and hits the shot, only to have the supposed easy put away fly over the baseline. The player then claims a let. The claim is not valid because the player forfeited the right to call a let by choosing instead to play the ball. The player took a chance to win or lose and is not entitled to a second chance.