Say you hit your tee shot into the trees. Fearing that it might be lost, you call for a provisional and grab another ball from the same sleeve. As a model of consistency, you then proceed to pump another ball into the same area as the first. Figuring one of them should be findable, you head into the trees to search.
Now comes the fun part: you come across both the balls you just hit and there is no way of knowing which one is the original and which one is the provisional. While it’s always a good idea to mark the second ball differently from the first so you don’t have this problem, it’s too late for that now.
According to Rule 18.3c(2), you must choose one of the two and treat it as the provisional – not the original. It’s now the ball in play and you lie three after adding a penalty stroke. Tough break, but hopefully it only happens to you once in your golf career. As a minor consolation, be sure to choose the one that gives you the best chance of escaping the trees on your next shot.
