Rules Guy: What do you do when you and your partner hit each other’s ball by mistake?

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My ball settled into thick rough a few yards in front of the green. I identified it and moved away as my opponent played his pitch shot. He bladed it, and his ball struck my ball, which popped up and returned to its original position. His ball caromed into a greenside bunker. I played my pitch shot onto the green; he went to play the bunker shot — and discovered that it was not his ball but mine, while I had mistakenly played his ball! We decided that he should play the ball from the bunker and then just switch back our balls. Was that correct?  
—Eric Bjankini, via email

In two words, heavens no. Consider, my good Mr. Bjankini: Why should your opponent play the wrong ball from the wrong place?

In match play, under Rule 6.3c, you lose the hole once you play the wrong ball — end of story.

Rules

In stroke play, things get a bit more complex. You get a two-stroke penalty and need to replay your shot from that original spot, only with the correct ball. Your fellow competitor also needs to place his ball in that spot and play from there as well, only without penalty, since he’s done nothing untoward (other than blade his prior shot).

As to what order you play in, since you’re both on the same spot, that’s for you two to work out.

And as to the lie potentially changing after the first player goes, Rule 14.2d has you covered. If the lie is altered before replacing the ball, the second player will place the ball on the nearest spot with a lie most similar to the original lie within one club-length of the original spot, no nearer the hole and still in the general area.

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