Rule 3.15

Rule 3.15
No person shall be allowed on the playing field during a game except players and coaches in uniform, managers, news photographers authorized by the home team, umpires, officers of the law in uniform and watchmen or other employees of the home club. In case of unintentional interference with play by any person herein authorized to be on the playing field (except members of the team at bat who are participating in the game, or a base coach, any of whom interfere with a fielder attempting to field a batted or thrown ball; or an umpire) the ball is alive and in play. If the interference is intentional, the ball shall be dead at the moment of the interference and the umpire shall impose such penalties as in his opinion will nullify the act of interference.

Rule 3.15 Comment: For interference with a fielder attempting to field a batted or thrown ball by members of the team at bat or base coaches, who are excepted in Rule 3.15, see Rule 7.11. See also Rules 5.09(b), 5.09(f) and 6.08(d), which cover interference by an umpire, and Rule 7.08(b), which covers interference by a runner.

The question of intentional or unintentional interference shall be decided on the basis of the person’s action. For example: a bat boy, ball attendant, policeman, etc., who tries to avoid being touched by a thrown or batted ball but still is touched by the ball would be involved in unintentional interference. If, however, he kicks the ball or picks it up or pushes it, that is considered intentional interference, regardless of what his thought may have been.

PLAY: Batter hits ball to shortstop, who fields ball but throws wild past first baseman. The coach at first base, to avoid being hit by the ball, falls to the ground and the first baseman on his way to retrieve the wild thrown ball, runs into the coach. The batter-runner finally ends up on third base. Whether the umpire should call interference on the part of the coach is up to the judgment of the umpire and if the umpire felt that the coach did all he could to avoid interfering with the play, no interference need be called. If, in the judgment of the umpire, the coach was attempting to make it appear that he was trying not to interfere, the umpire should rule interference.

DISCUSSION
Rule 3.15 deals with interference by authorized individuals on the field (commonly called "authorized persons") except the offensive team or umpires.

When an authorized person unintentionally hinders a player or touches a live ball, the ball is live and play continues.
Ball boy unintentionally hinders right fielder's attempt to field a fair batted ball.
http://mlb.mlb.com/news/article.jsp?ymd=20100428&content_id=9655812&fext=.jsp&c_id=hou&partnerId=rss_hou
Security guard unintentionally hinders player
http://mlb.mlb.com/video/play.jsp?content_id=21744837&c_id=mlb
First base coach is hit by throw and unintentionally interferes
http://mlb.mlb.com/video/play.jsp?content_id=12795303&topic_id=14873330


When an authorized person intentionally hinders a player or touches a live ball, the ball is dead and the umpire crew imposes penalties that will negate the interference.
Ball boys and ball girls intentionally touch live batted balls
http://mlb.mlb.com/news/article.jsp?ymd=20100615&content_id=11193500&vkey=news_mlb&fext=.jsp&c_id=mlb
Runners on first and second. Second baseman catches a pop up for second out and gives ball to ball girl. The ball girl tosses the ball into the stands. Once ball girl intentionally touches the ball, play is immediately dead. The umpires are instructed to award bases that will negate the interference. Interestingly, the umpire crew awards two bases to the runners for the ball girl throwing the ball out of play!
http://mlb.mlb.com/video/play.jsp?content_id=17335243&topic_id=9780550&c_id=mlb

It is not authorized person interference when a batted ball that has no chance of becoming fair is intentionally touched.
Ball boy makes catch on a foul line drive that has no chance of becoming fair or being caught
http://mlb.mlb.com/video/play.jsp?content_id=14455179