Read the Rule, Rizzo - Ball Boy Non-Interference

Chicago's Anthony Rizzo ran into an out due to a lack of rules knowledge concerning a Wrigley Field security guard's unintentional interference during a batted ball. Cubs batter Kris Bryant's double down the left field line led to a Cardinals' inning-ending out as Marcell Ozuna stayed with the play to throw the confused Rizzo out at home to end Chicago's 5th inning threat.

Types of Interference in the Outfield (or Infield)
Fan Interference: Perhaps it is apropos that we just wrote about fan interference, and reviewed Official Baseball Rule 6.01(e), which states that the penalty for spectator interference is: "the ball shall be dead at the moment of interference and the umpire shall impose such penalties as in his opinion will nullify the act of interference."
Related Post: Replay Rewind - Fan INT & Com Clarity (6/7/19).

Intentional interference on-field in Texas.

Unintentional Interference: Unlike the aforementioned fan interference that causes a dead ball immediately upon its occurrence, so-called unintentional interference can only occur as a result of a ball coming into contact with a person authorized to be on the playing field who are not uniformed personnel nor umpires—namely ball boys and bat girls, security guards, and police officers.

The rule reference is OBR 6.01(d), which states: "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."

The following articles illustrate additional considerations for officiating this brand of interference.
Related Post: When Unintentional Interference Turns Intentional (9/2/18).
Related Post: Ball Boy Interference: Judging Intent of Non-Team Persons (4/12/13).

Culbreth ejects Collins over a similar play.

Prior Play: In 2017, Fieldin Culbreth ejected Terry Collins for arguing an unintentional interference foul ball call by HP Umpire Roberto Ortiz when a Mets batboy attempting to avoid a foul ball unintentionally collided with Mets fielder Wilmer Flores.

Because the batboy did not intentionally initiate contact with Flores (nor with the ball), the proper call was made: unintentional interference. To be clear, unintentional interference can occur when a batboy-type person makes contact with a batted or thrown ball, or with a player.
Related Post: MLB Ejection 057 - Fieldin Culbreth (1; Terry Collins) (6/1/17).

We diagrammed this play in 2018.

Analysis: So there you have it. When Cubs batter Bryant hit a double down the left field line, the bounding ball bounced into the Cubs' left field warning track security officer, who himself was attempting to avoid the ball (perhaps due to his proximity to the ball girl, he jumped back and to his left instead of forward and to his right).

By giving up on Friday's play in Chicago, Rizzo demonstrated his rules ignorance—as did the Cardinals' Paul DeJong—while Ozuna, who did raise his arm, nonetheless kept running after the ball and threw home in baseball's version of playing to the whistle as Rizzo jogged into an out at the plate, a simple call for HP Umpire Mark Carlson.

Maddon speaks to U3 Gorman.

Mechanics: The one thing we didn't see that we could have is 3B Umpire Brian Gorman giving a "safe"-type mechanic to communicate his ruling that the interference by an authorized on-field person was unintentional. In general, "safe" tells players and teams that play is alive and no infraction of the rules has occurred. Other than "safe," the only other mechanic, if one were to be given, here would be a signal declaring the ball dead (due to intentional interference, which this was not). Thus, "safe" and no mechanic at all signify the exact same thing: the ball remains live and in play.

SIDEBAR: Unlike fan interference, unintentional interference is not reviewable. Despite Joe Maddon putting his hands up to hold the umpires, this play could not be referred to video.

Compare to Intentional Interference by On-Field Persons:

This is intentional interference.

By rule, plays like Friday's sequence in Chicago are deemed "unintentional interference," which results in the ball being kept alive and in play. It is important from a rules standpoint to understand that "unintentional interference" doesn't refer directly to whether the non-team authorized person actually intended to interfere with a live ball, but to the result of this person's action.

In other words, this rule uses a different definition of "intentional" than, say, the rule about a pitcher intentionally throwing at a batter.

For instance, a ball attendant sitting on a stool who fields a ground ball, believing that the ball was foul despite the umpire's ruling that the ball was fair, has not intended to interfere per se—but, by virtue of this person's intentional act in fielding the ball, this person has indeed committed intentional interference.

As OBR 6.01(d) states, if the interference by such an authorized non-uniform person (which, again, includes a non-roster ball attendant in uniform) is an intentional act—as in this person intentionally seeks to touch the ball—then the penalty is similar as to that for fan interference: dead ball and nullify the act.

Until then, read the rules and don't stop playing before the ball is declared dead.

Video as follows:

Alternate Link: Rizzo gives up on the play, resulting in an out in Chicago (STL)