From BR Bullpen
Fan interference , also called spectator's interference, is a form of interference caused by fans who enter the field of play for the purpose of retrieving a baseball. In the case of a foul pop-up, umpires may award an out if it is deemed that a fan interfered and the player would have made a catch. In the outfield, when a fan reaches over the fence to interfere with a fair ball, the player is awarded a double. Fans who interfere with batted balls in play are usually removed from the ballpark.