As described by the New York Times:
He jogged after yet another errant putt and shockingly swatted the moving ball back toward the hole with his putter. It was one of dozens of missed putts in the last three days by Mickelson, and his response to yet another disappointment was familiar to any golfer.
He snapped. It was an act of frustration.
For such a serious breach of golf’s rules, Mickelson could have been disqualified from the championship. In a technicality, or a generous rules interpretation by the United States Golf Association, Mickelson was assessed only a two-stroke penalty and allowed to play on.
But the bigger damage came after Mickelson’s third round at the Shinnecock Hills Golf Club ended with a discomforting score of 81, which left him a humiliating 17 over par for the tournament.
It was at this moment that Mickelson beseeched his peers, the greater golf community and his legion of fans to believe that his slap at a moving ball was actually a calculated, astute use of the rules — just another way for a PGA Tour veteran to save a few strokes: