When a competitor plays your ball, what do you do?
Why did Mahan and Donaldson take drops instead of placing their balls? Couldn't they figure out where each other hit from and recall what the lie was like or is there something else in the rules that I am missing?
15-3. Wrong Ball
b. Stroke Play
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 astroke 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.
(Placing and Replacing – see Rule 20-3)
20-3. Placing And Replacing
b. Lie of Ball to be Placed or Replaced Altered
If the original lie of a ball to be placed or replaced has been altered:
(i) except in a hazard, the ball must be placed in the nearest lie most similar to the original lie that is not more than one club-length from the original lie, not nearer the hole and not in a hazard
c. Spot Not Determinable
If it is impossible to determine the spot where the ball is to be placed or replaced: