You're 236 yards out on a par 5, and you hit your 3-wood flush. But as it tracks toward the green–DOINK!–it hits a power line and bounces into the rough. What do you do? According to Decision 33–8/13 in the Rules of Golf, a course can enact a Local Rule allowing for the replay of the previous shot without penalty.
Note: You don't have the option of playing the ball from its new position. However, if the course doesn't have this Local Rule, play the ball as it lies.
Q: You think the ball you just hit is in a water hazard, but you're not sure. Can you play a provisional ball?
A: Yes, but if it turns out the ball went into the hazard, the provisional ball must be abandoned, and you must proceed with the original ball.
The ball moved, but is it a penalty?
First, know the definition. A ball is deemed to have moved only if it comes to rest in a new place. But a ball that oscillates has not "moved." If the ball moves after you've addressed it (outside a hazard, "address" means taking your stance and grounding your club), then add a penalty stroke to your score and put the ball back. There's no penalty if it is moved by an outside agency (a spectator, squirrel, etc.) or as a result of actions such as searching for it in a hazard, removing an obstruction or a loose impediment on the green. But you still have to replace it. / Shuhei Matsuo