Russell Henley was penalized eight shots -- yes, eight -- on Saturday after the second round of the 2019 Mayakoba Golf Classic when it dawned on him that he violated the One Ball Rule.
Henley was signing autographs for fans at El Camaleon Golf Club in Playa del Carmen, Mexico, when he realized he had a ball in his bag different from the model he typically plays (Pro V1x). He used the ball during the round, from hole Nos. 9-12, which is a violation of Rule 20-3, which covers rules not explicitly covered under the Rules of Golf.
The One Ball Rule and its enforcement falls under Rule 20-3 because it is not explicitly laid out in the rule book. It's actually a Local Rule under the Tour's Hard Card, falling under G-4.
The One Ball Rule requires a golfer to use the same manufacturer and model of golf ball, as well a golf ball with the same color throughout a round. That means a golfer can't switch between white, yellow, pink, etc., during a round in the event it would prove advantageous because of a change in conditions.
If a golfer violates the One Ball Rule, they can be penalized in a variety of ways by the tournament organizer. However, the penalty is up to the tournament committee. Before 2019, the USGA recommended a two-stroke penalty for each hole on which the rule is violated, with a maximum of four penalty strokes. Under the new Rules of Golf instituted in 2019, the general penalty is applied -- meaning a two-stroke penalty for every instance in which it happens. In match play, the USGA recommends a loss of each hole on which the rule is violated, with a maximum of two holes lost.
Henley was penalized eight strokes. Instead of signing for a 2-under 69, Henley had to sign for 6-over 77, and he will miss the cut.