What is the consequence for a runner who leaves second base too soon on a caught fly ball and is on third base when the ball becomes dead?

When a runner leaves a base too soon on a caught fly ball, the proper procedure involves the possibility of an appeal by the defensive team. If the runner advances to another base—like third base—and the ball becomes dead, the defensive team can appeal the infraction. In this situation, the umpire will uphold the appeal, resulting in the runner being called out for leaving second base too early.

This rule exists to ensure that runners adhere to the prescribed base-running procedures when a fly ball is caught, which is crucial to maintaining the integrity of the game. The appeal process allows the defensive team a chance to enforce this rule. Thus, in this scenario, the runner would indeed be declared out upon a proper and successful appeal.