Rule 3.04

Rule 3.04

A player whose name is on his team’s batting order may not become a substitute runner for another member of his team.

Rule 3.04 Comment: This rule is intended to eliminate the practice of using so-called courtesy runners. No player in the game shall be permitted to act as a courtesy runner for a teammate. No player who has been in the game and has been taken out for a substitute shall return as a courtesy runner. Any player not in the lineup, if used as a runner, shall be considered as a substitute player.


DISCUSSION
The practice of courtesy runners was allowed until 1950. However, in 1952 a courtesy fielder was allowed. The Chicago Cubs were playing the Pittsburgh Pirates and in the top of the ninth Pirates catcher Clyde McCullough was injured and could not continue. The Pirates two other catchers, Eddie Fitzgerald and Joe Garagiola, had already been used in the game as pinch hitters. With the approval of Cubs manager Phil Cavarretta, Fitzgerald was allowed to replace McCullough. The Cubs won the game 4-3. Under the playing rules in effect since the 1950 season, this illegal substitution should not have been allowed.
http://www.retrosheet.org/boxesetc/1952/B08102PIT1952.htm

Eddie Fitzgerald was involved in a very funny play in the early 1950s. The Phillies were playing the Pirates and Pittsburgh pitcher Bill Werle induced the Phillies' Bill Nicholson to hit a high pop-up in the middle of the diamond. Werle directed traffic and yelled, “Eddie’s got it! Eddie’s got it!” The ball landed untouched in the grass. No one believed what they just saw… especially first baseman Eddie Stevens, catcher Eddie Fitzgerald, and third baseman Eddie Bockman.