Let's say Alexei Ramirez hits a scorching drive down the third base line. The umpire immediately signals foul ball. Alexei trots back to the dugout. The play gets challenged, and it becomes apparent that the ball should have been called fair and the call is reversed. What happens? Does Ramirez get first base? Second base? Does he have to run out of the dugout to first before the pitcher can get the ball to the first baseman? If he never touches first, can the manager of the opposing team challenge so they can try to get him out at first?
There are so many instances where the play stops because of a call and you have no idea what would have happened after the play with the opposite call.
De Aza is on 2nd. Viciedo hits a drive to center. The centerfielder traps the ball, but it is ruled a catch. It's not deep enough for De Aza to tag to 3rd, but he is leading off far enough that he could easily make third if there is no catch. The play is challenged and overturned. Does De Aza get to go to 3rd?
What is Mind? -- Doesn't Matter!
What is Matter? -- Never Mind!