As if the U.S. Open  isn't hard enough, Hunter Mahan and Jamie Donaldson made it much more difficult on themselves during Friday's second round.
Playing the 18th hole -- their ninth hole of the day -- they both found the fairway... and that's where the trouble began.
Both players were playing a Titleist ProV1x, and failed to properly identify their own ball before hitting their approach shots.
Once they arrived at the green and marked their balls, they realized that they had hit each other's ball onto the green. The returned to the spot of their approach shots to take a penalty drop (a two-shot penalty) and played the hole out from there. Both players made a double-bogey 6.
Had they played out the hole and teed off at No. 1 -- the next hole -- Mahan and Donaldson would have been disqualified.
Here is Rule 15-3:
If a competitor makes a stroke or strokes at a wrong ball, he incurs a penalty of two strokes. The competitor must correct his mistake by playing the correct ball or by proceeding under the Rules. If he fails to correct his mistake before making a stroke on the next teeing ground or, in the case of the last hole of the round, fails to declare his intention to correct his mistake before leaving the putting green, he is disqualified. Strokes made by a competitor with a wrong ball do not count in his score. If the wrong ball belongs to another competitor, its owner must place a ball on the spot from which the wrong ball was first played.
Exception: There is no penalty if a competitor makes a stroke at a wrong ball that is moving in water in a water hazard. Any strokes made at a wrong ball moving in water in a water hazard do not count in the competitor's score.
Here are some tweets about the incident: