You don't see this every day. Hunter Mahan and Jamie Donaldson both incurred two-stroke penalties in the second round of the U.S. Open because they played the wrong ball -- the other's ball.
Playing the par-4 18th, Mahan and Donaldson played their approach shots into the hole. Once they got there, they realized they had played each other's golf ball by mistake. The penalty is two-fold: two strokes added to each player's score, then they have to go back and replay their ball from the original spot where they should've played it in the first place. In this case, that meant the players had to return to the fairway and drop approximately where their balls had stopped.
Rule 15-3 of the Rules of Golf states:
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.
The mistaken approach shots each hit didn't count, leaving them each hitting their fourth after the correction. They each hit the green and two-putted for double-bogey 6.