Unhappy Birthday: Twitter stunned by Mickelson's incredible mental gaffe
Streeter Lecka / Getty Images Sport / Getty

It's Phil Mickelson's 48th birthday on Saturday, but after a bizarre incident on the back nine at Shinnecock Hills, it's likely a day the legendary golfer will want to forget.

Walking onto the 13th green at 4-over for his round, Mickelson blew a putt past the hole and shockingly ran after the ball to send it back toward the cup while it was still moving.

That instantly results in a two-stroke penalty, and led to Mickelson carding a 10 on the par-4. The five-time major champion's decision to blatantly break the rules of golf because he wasn't pleased with how things were going was stunning, and Twitter reacted accordingly.

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Unhappy Birthday: Twitter stunned by Mickelson's incredible mental gaffe
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