The penalty for hitting a moving ball is a two-stroke penalty, and Mickelson was provisionally assessed with a 10 at the par-four hole.
"Phil didn't purposefully deflect or stop the ball", USGA official John Bodenhamer explained, adding that the determination was unanimous.
Yeah, uh, you can't do that.
Mickelson later said he did not mean the shot to be disrespectful, saying there was "no question" the ball was going to roll off the green, and he chose to take the penalty rather than the alternative.
It was not immediately clear what was Mickelson's motive in striking the ball.
However, Berger's score, which equalled the lowest of the week so far, proved that it was possible to make up ground, the world number 43 carding six birdies and two bogeys to move just outside the top 10.
There was some question as to whether the USGA could disqualify Mickelson under Rule 1-2, which outlaws playing a moving ball to gain a "signifiant advantage".
As he walked off the green, he could be seen smiling and talking to Johnston, who also was smiling. "I've never seen anything like that from a world-class player in my life".
It's the type of thing you'd see a stroppy youngster perform, frustrated at their putt not going into the hole. "But it was just a moment - I think it's just a moment of madness. It was unusual, no one ever has those thoughts, it just happens".
Lefty purposely hit his birdie putt on No. 14 wide of the hole and up a slope behind the cup. His swat sent the ball off the hole.
What's unclear is why Mickelson lost his mind.
Mickelson's actions were reminiscent of John Daly hitting a moving ball at Pinehurst No. 2 in the 1999 U.S. Open.
Phil Mickelson of the United States plays a shot during practice rounds prior to THE PLAYERS Championship on the Stadium Course at TPC Sawgrass on May 8, 2018 in Ponte Vedra Beach, Florida.