A player hits a ball presumably out of bounds.  The player hits a provisional ball and lands on the green.  The player then finds his first ball in play.  He hits his first ball on the green.  The player then picks up what he thought was his provisional and pockets it. When the other player notices that the wrong ball was picked up.  The player then returns the pocketed ball to what is deemed the correct position on the green.  Does the player who picked up the other players ball incur a penalty?  The players were playing stroke play.

Hi Scrotty13,

In stroke play, fellow-competitors are outside agencies with respect to one another.  As such, Rule 18-4 applies and there is no penalty to either player so long as the ball is replaced.

However, in match play, the player and opponent are not outside agencies, Rule 18-3 applies, and the player that lifted the opponent’s ball would incur a one stroke penalty.

Thank you for you interest in the Rules of Golf - regards,