Though he might not know the date, Phil Mickelson knows exactly what happened 20 years and four days ago.

That’s when John Solheim allowed the USGA to save face. He dropped the $100 million lawsuit filed by the company that his dad founded, Karsten Manufacturing Corporation, makers of the controversial Ping Eye 2 irons. Previously, the USGA had tried to ban the Ping Eye 2 — the club Mickelson is drawing criticism for using this week — and it’s new groove design, called the “U-groove.” The USGA said it was unfair how much spin it put on the ball, especially out of long grass.

So Solheim’s father, Karsten, took the USGA to court. He said it violated anti-trust laws that his clubs — which already had sold more than a million sets — could be banned from competition. If the suit had gone to a jury, and the verdict came back that the Solheims were right, it would have undercut all authority held by the USGA as the sport’s governing body — a title it held since its inception in 1895.

The USGA caved, allowed all Ping Eye 2 irons made before March 31, 1990 to be legal, and told the pesky Solheims to stop making their clubs so much darn better than everyone else.

William C. Battle, then-president of the USGA, trumpeted for all to hear that the compromise was a “reaffirmation of the USGA as the sole rule-making body” in United States golf.

That arrogance has never left the USGA, and it has never been more evident than when they tried to impart the same ban on U-grooves again, starting this season.

With technology at the level it’s at now, it really wasn’t a big deal. In a plethora of tests that took years to conduct, the different grooves showed to have minimal affect on how much a pro would be able to spin the ball. And the USGA postulated that players could always change the type of ball they were playing if they wanted more spin.

Now U-grooves are banned, and for the first three weeks of the 2010 season, the only question was if it was affecting the pros at all.

But how about that settlement made 20 years ago? The one that says Ping Eye 2 irons made before March 31, 1990 are legal?

Yup, Mickelson remembered, and he went out and found that Ping wedge with big, gapping U-grooves, too close together on the face of a wedge that is now bent to 64-degrees. He hit a flop shot on Thursday from rough behind the 17th green on Torrey Pines South Course that went straight up in the air, traveled over the two feet of green he had to work with, and stopped right next to the pin. Stopped dead.

Was it the grooves that did it, or Mickelson’s immense skill? The USGA would like to believe the former, everyone who saw the shot believes the latter, and the only definitive thing is that Mickelson kicked it in to save par and continued along with his legal — but illegal — wedge in his bag.

Later that afternoon, Scott McCarron, a three-time winner on the PGA Tour and consummate professional, said Mickelson was exploiting a loophole in the rules by playing the wedge.

“It’s cheating,” he said. “And I’m appalled Phil has put it in play.”

Three players other than Mickelson this week at Torrey Pines have the same, or very similar, wedge in play: Chris Couch, Tim Herron and Mark Calcavecchia.

“It’s a terrible rule,” Mickelson said. “To change something that has this kind of loophole is nuts. But it’s not up to me or any other player to interpret what the rule is or the spirit of the rule.”

Is Mickelson going against the ambiguous “spirit of the rule,” as McCarron later suggested? Probably. But what kind of tainted spirit does a rule have if there is such a gaping hole in it?

When the USGA tried to ban U-grooves the first time around, it found itself in court with pie on its face. By the grace of the Solheim family, it escaped unharmed, still the conservative Blue Blazer Juggernaut that ruled over the game with a steel fist and warm heart.

If Mickelson wins today with a Ping Eye 2 wedge in his bag, he will be throwing a new pie in the face of the USGA. And this time, it knew it was coming.

After 20 years and four days, the USGA still can’t get out of its own way.