Is this a one golf stroke penalty situation?

Dear Golf Nut, I have a golf rules question for you. Recently, during a friendly round of golf with my usual foursome something unusual happened with my golf ball. We were about half way through the round when I noticed that I was playing the wrong golf ball. It was one like mine but had a mark on it from a permanent marker. When I mentioned it to my partners, we suddenly realized that somehow one of the golfers in our group and i had accidentally started playing each others’ golf balls. We don’t know how or when this happened and really didn’t know what to do. Should each of us taken a one penalty stroke or no penalty? Thanks for your help.

Sometimes when you are checking the rules of golf, you have to dig a little to get an answer. In the official rules of golf there is a section for each rule called decisions. This is where the USGA has made a ruling on a particular or unusual situation that has come up on the golf course. Your question is answered in one of these decisions.
Decision 15-1/2 addresses this situation. It states that if it cannot be determined that you switched balls during the play of a hole, you get the benefit of the doubt. As a result the Rules of Golf treat this particular situation just like you had accidentally switched balls without realizing between holes rather than during play of a particular golf hole. So no
penalty is imposed on you or your partner.
This is a situation that you don’t see that much with the Golf Rules. What I would call a little compassion and understanding. So many times, the rules are unbending, even with inadvertent mistakes. So thank the golf gods on this one and thanks for the question!