Here's my recent rules violation story: Two weeks ago I played in a 2 man best ball tourney with one of my buddies.
They had an odd number of teams, and we were last to sign up, so the pro sent us out with one of the kids from the bag room as a marker
(he played with us and kept our score). On the 4th hole we both hit good drives to a blind landing area, and our balls
ended up perfectly in line with each other 10 yards apart. I hit first and knock it on to about 20 feet. My partner mis-hits his shot, but
ends up just short of the green. As I'm walking toward my ball on the green, he's getting ready to chip and I hear an "Oh Shit".
Before he said anything more, I go to mark my ball and realize I had hit his ball, and he had hit mine. We were playing the same ball, different number, and I even had a mark on my ball. BUT, there was nobody in front of us so we were playing fast, and we had a side conversation going in between shots that distracted us both. He is consistently longer than me, so I was a dumb ass and assumed the shorter ball was me (I remember looking at the ball quickly, but it was sitting in a way where the number wasn't visible). After I hit, he didn't check the other ball closely, assuming that I had just hit my ball, and the kid playing with us was on the other side of the fairway.
SO, instead of making an easy 4, possibly 3, we both take a 2 shot penalty and had to re-play from the correct spot. We were both a
little rattled, and both made bogey when we went back and re-played the second shots, plus the penalty, for a triple. We ended up 1 over for the day with 15 pars, 2 birdies and the triple. We would have tied for 2nd if we made par, tied for first with a birdie. Instead we finished 4th, out of the money. Those kind of mistakes ALWAYS come back to haunt you.
I've hit the wrong ball in casual rounds before, but that's the first time I've ever hit done it in over 40 years worth of tournaments. I doubt it would have happened if we had been paired up with 2 other guys playing the tournament. It took away the tournament "feel" playing with the kid as a marker, and we lost focus.