Well, we could start with the whining Cubs fans who keep yapping about fan interference. If the ball is in the stands it is NOT fan interference. If a fan reaches out into the field of play to grab a ball that would be caught in the field of play (like the infamous Yankee home run from several years back) that would be interference. But when a fan touches a ball that is beyond the railing and in the stands, preventing a player from reaching into the stands to make a play, that is simply a case of a ball out of play.