Playing the wrong ball is a bitch. I've done it twice, once in a tournament. I make an effort to ID my ball every time. It's the time you're sure it's your ball that it turns out it isn't. You have 5 minutes to look for lost balls, I believe. If it is declared lost, you hit the provisional you should have hit after the tee shot, stroke and distance penalty. If you didn't hit a provi, you get to walk back to the tee box and hit another one in front of the people waiting to tee off. That's the USGA rule. Our league has a rule that if a ball is lost in the rough, you just drop it where you thought it landed with a one stroke penalty. Speeds up play.