From SmashWiki, the Super Smash Bros. wiki
DUMMY is a stage in Super Smash Bros. Melee which can only be found via Action Replay. When activated in the debug menu, it crashes the game because the game does not specify a pointer for background music. This can be rectified through another hack that can force the game to present the stage without background music. DUMMY is primarily composed of empty space over a black background, save for a lone invisible platform in the center that can only be seen using the debug menu.
Unusually, while the stage has a standard set of blast lines, they are located extremely far from the stage's center; as a result, the game will crash before players come in contact with them, due to the game being unable to process the character's position on the screen.
DUMMY's purpose is unknown, and nothing in the game gives any suggestion as to what it was intended to be.