WATCH: Lexi Thompson Penalty Video

Pro golfer Lexi Thompson lost the ANA Inspiration after a bizarre twist: She received a four-stroke penalty for “incorrect ball placement” after a TV viewer belatedly spotted the error.

You can watch the video above.

Thompson was leading when officials informed her of the penalty on April 2 – for a ball placement which had occurred the day before. A television viewer had tipped them off to it, and that has a lot of people on social media very upset. It’s also drawn the ire of Phil Mickelson; according to The Sporting News, he believes the Thompson penalty should be reversed. “I think that she should be given the trophy,” the pro golfer said, The Sporting News reported.

According to ESPN, Thompson appeared to be “cruising to her second major championship when Tour rules officials informed her of the penalty with six holes to play.”

The LPGA released a statement explaining its decision:

So Yeon Ryu went on to win.

In a post on Instagram, Thompson said the move that drew the penalty was not intentional.

Twitter users were generally angry at tournament officials for imposing the penalty. Many thought it was unfair because it was belated and spotted by an eagle-eyed TV viewing fan. Others mocked the anonymous viewer.

According to CBS Sports, “someone emailed the LPGA on Sunday about a moment that happened in Thompson’s Saturday round, and she was docked four strokes, which turned her Saturday 67 into a 71.”

CBS reported that the moment that caused the penalty “came at the par-3 17th hole on Saturday. Thompson appears to have moved her ball slightly after marking it.”

The viewer “emailed the LPGA on Sunday,” CBS reported, “and Thompson was told about the penalty during the middle of her round and that she would be docked two strokes for the movement of the ball and two more for signing an incorrect scorecard.”