The Mystery Zone glitch is a glitch in Brawl that takes place during the Home-Run Contest. Normally, knocking the Sandbag to the left of the platform would result in zero distance and end the game. However, using a second player, Sandbag can be hit out into the field before being whacked far left with the Home-Run Bat. Performing this glitch will allow the Sandbag to travel left and see what the stadium looks like left of the platform.
As the camera pans left, the blank terrain can be seen. There are no tiles or distance markers on the ground, probably because nothing was intended to be in this area. Surprisingly, there is a wall about 200ft to the left that Sandbag will hit and bounce off of, and even makes a sound. (This sound is the same sound effect heard when Sandbag hits the normal use force field.) It is possible to go over this wall if Sandbag is flying high enough. The Stadium and ground ends at about -200ft, and after that only the sky background will be shown. Since there won't be any ground, Sandbag will not be able to land. At -1200 feet, the sky will end and Sandbag will be flying through black emptiness.