Updated

Whose ball is that anyway?

Playing partners Hunter Mahan and Jamie Donaldson finally came up with the right answer Friday — too late to do either much good.

Caught up in one of those mistakes you're more likely to see at a junior tournament than the U.S. Open, each played the other's ball after teeing off on No. 18 — though they were playing their ninth hole of the day. Neither realized the mistake until they arrived at the green. Both got slapped with a two-shot penalty and missed the cut.

"It was 100 percent on me. I was the first one to the ball," said John Wood, Mahan's caddie.

He went on to explain that both players use "kind of an interesting slash across the number, which is not very common" to mark their balls.

"The only thing I can think of is I saw a slash and assumed it was Hunter's. That's all I can think of," Wood added. "But still, it's hard to believe. I don't believe I did something that epically dumb, but I did."

Mahan shouldered plenty of responsibility as well.

"I mean, I don't know, it was one of those things I couldn't explain to you," he said. "Off the tee, it looked like that's where my ball should have been, and I couldn't explain to you how it ended up where it did."

After sorting out the confusion on the green, both players walked back up the fairway and re-played the shot from the correct spot. Each wound up making a 6. Had they teed off on the next hole without doing so, both would have been disqualified under Rule 15.3.

Mahan, who was 2 under going into the hole, composed himself enough to birdie the next one. But he wound up shooting 37 on his back nine for a 72, and won't be around for the weekend after finishing at 6 over for two rounds.

Donaldson, who left the course before talking to reporters, wouldn't have made the cut in any case. He shot 43 on the front, 38 on the back, 81 for the round and 11 over for his two days.

The debacle was televised on ESPN as commentators and former players Paul Azinger and Curtis Strange, a two-time U.S. Open champion, commiserated throughout.

"They're gonna kick themselves the rest of the day for that," Azinger said.

"Everybody should do it once," Strange concurred, "because you will never do it again."

Oddly enough, Mahan acknowledged afterward that he had — "maybe one time before. It's a fluke thing."

Wood has been caddying for 17 years and called this mix-up his first.

"You can't imagine yourself doing something colossally as stupid as that," he said, "but I did it. I won't forgive myself very soon after this."