In which scenario is a player deemed an illegal substitute?

A player is deemed an illegal substitute when a player acts as a courtesy runner for a player after previously running for another player in the same game. In this scenario, the individual has already participated as a runner for one player, and then they cannot legally assume the role of runner for another player during the same game. This rule is designed to keep the game fair and prevent any advantage that could be gained by having the same runner participate multiple times.

In contrast, when a player is replaced by a teammate during injury time, this is a legitimate substitution as it falls within the guidelines for handling injuries. Similarly, a player changing positions without notifying the umpire does not constitute an illegal substitute; however, it is required that umpires be informed of such changes for the sake of record-keeping. Lastly, if a player is ejected and immediately replaced by a new player, this is a standard procedure that is permitted within the game rules, as ejections necessitate a substitute to maintain the team's roster.