UPDATE: The USGA has released this tweet and a statement citing Rule 14-5 instead of Rule 1-2.:
During play of the 13th hole Phil Mickelson made a stroke on the putting green at the time his ball was moving. As a result, he incurred a two stroke penalty for a breach of Rule 14-5. His score for the hole was 10.— USGA PR (@USGA_PR) June 16, 2018
Rule 14-5 reads as follows and results in a two-stroke penalty:
SOUTHAMPTON, N.Y. — Throughout his career, Phil Mickelson has hit some of the craziest shots you've ever seen. But he's never done anything like this.
As he played the 366-yard par-4 13th hole on Saturday, Mickelson, 10 over par (and four over his last five), ran into some trouble. After hitting the fairway with his tee shot, Mickelson's approach shot ran over the back of the green, and then his chip scampered past the hole and off the front edge. He chipped it from there to a point 18 feet past the hole. Then things got really bizarre.
As his bogey putt rolled past the hole, it started to pick up speed. As it headed toward a steeper slope, Mickelson broke into a jog, caught up to the ball and hit it back towards the hole with his putter before it could roll all the way down the slope. You can see the video below.
Phil Mickelson just did something you do if you're four years old and playing mini golf for the first time. pic.twitter.com/k0AxplqdMg— Sean Zak (@Sean_Zak) June 16, 2018
USGA Rule 1-2, 'Exerting Influence on Movement of Ball or Altering Physical Conditions' reads as follows:
A player must not (i) take an action with the intent to influence the movement of a ball in play or (ii) alter physical conditions with the intent of affecting the playing of a hole.
The penalty for Breach of Rule 1-2 is a two strokes (or a loss of hole in match play). The kicker is that the rule includes a clause for certain violations: "In the case of a serious breach of Rule 1-2, the Committee may impose a penalty of disqualification," the rule reads.
It also includes this note, suggesting that if Mickelson is deemed to have gained a "significant advantage" he could be subject to the disqualification: "A player is deemed to have committed a serious breach of Rule 1-2 if the Committee considers that the action taken in breach of this Rule has allowed him or another player to gain a significant advantage or has placed another player, other than his partner, at a significant disadvantage."
Mickelson's score was initially recorded as a 9 and later changed to a 10. This post will be updated as GOLF.com receives more information.