Download this quick reference guide Accidental Movement of a Ball on the Putting Green and or watch this video from the USGA to learn more about this rule: Video: Accidental Movement of the Ball on the Putting Green
Rules 18-2, 18-3 and 20-1 are modified as follows:
When a player’s ball lies on the putting green, there is no penalty if the ball or ball-marker is accidentally moved by the player, his partner, his opponent, or any of their caddies or equipment.
The moved ball or ball-marker must be replaced as provided in Rules 18-2, 18-3 and 20-1.
This Local Rule applies only when the player’s ball or ball-marker lies on the putting green and any movement is accidental.
Note: If it is determined that a player’s ball on the putting green was moved as a result of wind, water or some other natural cause such as the effects of gravity, the ball must be played as it lies from its new location. A ball-marker moved in such circumstances is replaced.”