I played 9 holes with friend recently. We were both playing the same brand of ball with mine having my standard red mark on the first letter of the brand name. When my friend would hole out I would pluck my friend's ball from the hole since he is almost 20 years my senior and has back problems. Often he would then make a few practice putts and I would toss the balls back to him.
On one hole we each teed off, his ball on the right side of the fairway and mine on the left. When I went to hit I noted I had teed off with his ball. He had done the same with mine. It is possible we had hit the wrong ball on the previous hole but not likely. I think we switched balls as part of my getting his out of the hole, etc...
Which gets me to the point of this posting, if this were a tournament or match are we DQ'ed, do we take a penalty under the assumption we played the wrong ball previously or just finish the hole and swap balls again? I think in a tournament, even though it was probable the switch occurred after we both holed out, that we would be DQ'ed because there was a possibility we had hit the wrong ball on a previous hole and failed to correct the error prior to teeing off.
In reality we were just playing for fun and it is the off season in MI so the scores don't count toward our handicaps anyway. We just swapped balls after the hole.