Originally Posted by
Kilroy
Rule 18. Ball at Rest Moved
18-3. By Opponent, Caddie or Equipment in Match Play
b. Other Than During Search
If, other than during search for a player's ball, an opponent, his caddie or his equipment moves the ball, touches it purposely or causes it to move, except as otherwise provided in the Rules, the opponent incurs a penalty of one stroke.
If the ball is moved, it must be replaced.
18-4. By Fellow-Competitor, Caddie or Equipment in Stroke Play
If a fellow-competitor, his caddie or his equipment moves the player's ball, touches it or causes it to move, there is no penalty. If the ball is moved, it must be replaced.
(Playing a wrong ball - see Rule 15-3.)
Note 1: If a ball to be replaced under this Rule is not immediately recoverable, another ball may be substituted.