It plopped down about 20 feet right of the flag, hopped a couple of times, then rolled.
Mickelson wound up making double bogey at Shinnecock Hill's infamous 'Redan' hole, his first major stumble in pursuit of his second straight major.
By the time Saturday was done, so was Lefty's lead.
Closing with two straight bogeys, he settled for a 72 and will go to the final round two strokes behind leader Retief Goosen.
But Mickelson never lost that aw-shucks grin. He was down but not beaten, still very much in the running for his second straight major.
'Although I bogeyed the last two, it certainly doesn't change the way I look at the round,' he said. 'I felt like I fought really hard.'
Sure, Mickelson would prefer to have the lead, but at least he didn't get swept away on moving day.
Vijay Singh, who's been playing as well as anyone in the world, fell from contention with a 77. Tiger Woods failed to make a charge, struggling to a 73.
Mickelson arrived at the seventh tied for the lead with playing partner Shigeki Maruyama and Jeff Maggert. Up ahead, Maggert and Fred Funk were still on the green, giving Lefty a chance to watch and learn.
He would have been better off looking the other way.
Funk was playing his second shot from the back left fringe, but his chip stopped far short of the flag and rolled backward a couple of feet. He wound up with bogey.
Mickelson then teed off, hitting a shot that would have worked beautifully in another time, another place. But it tumbled off the steeply sloping green at Shinnecock - not far from the spot where Funk's ball stopped.
'I struck it great,' Mickelson said. 'But I knew it wouldn't hold the green.'
Remembering Funk's chip, Mickelson got himself in trouble. He struck his too firmly, winding up 8 feet above the hole and facing a treacherous downhill putt.
'I ended up giving it a little bit extra to make sure it got up the hill,' Mickelson said. 'I think I probably could have gotten a 4 or maybe even a 3 if I hadn't watched the group in front of me.'
He barely touched the par putt, but the ball rolled past the cup and refused to stop, creeping along slowly with Mickelson right alongside.
As soon as it paused, Mickelson quickly stuck a marker in the ground and snatched up his ball, taking no chances that it would start up again. Still, he missed a tricky 12-footer coming back and took double bogey.
'You could make bogeys out here,' Mickelson said. 'It's doubles that hurt.'
Mickelson recovered, stringing together seven straight pars, then went back to the top of the leaderboard with a 20-foot birdie putt at 14.
But Shinnecock Hills got the last laugh. Mickelson buried a 5-iron in the bunker at 17, blasted out to 12 feet and missed the putt. Then came 18, where a jittery putter - Lefty's problem at past majors - made an untimely return. He missed a 4-footer to save par, the ball curling around the edge of the cup.
That knocked Mickelson out of the final group with Goosen, who will instead play with fellow South African Ernie Els. Mickelson and Funk will be in the next-to-last group, perhaps commiserating about their misfortune at No. 7.
Not that they had anything to be embarrassed about. Redan was the toughest hole on the course Saturday, giving up one birdie while doling out 27 bogeys and three double-bogeys.
Mickelson played in the last group on Sunday at the Masters, where he shot 31 on the back nine and sank an 18-foot birdie putt on the last hole for his first major championship.
Els finished one stroke back despite shooting 67.
'We won't have that here,' Mickelson said. 'What we will have is guys outlasting each other, who can make the most pars before somebody bogeys, that type of thing. That's a fun way to play, too.'
Especially when you're the fan favorite.
Lefty has a two-year love affair with the Long Island crowds, who remember him coming up short in the Open at Bethpage two years ago. They want him to finish the job at Shinnecock Hills.
'It's been very flattering,' Mickelson said. 'The people here are just so much fun to play in front of. I've been having a great time.'
Well, except at No. 7
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