DUMMY is a stage in Super Smash Bros. Melee which can only be found via Action Replay. When activated in Debug Mode, it normally crashes the game, though another hack can force the game to present the stage. It merely consists of empty space, save for a lone invisible platform in the center that can only be seen using the debug menu.
DUMMY, with Link standing on the lone invisible platform of the stage
A somewhat popular technique used in DUMMY is to add blast lines, which the stage normally does not have, and then activate an infinite-double-jumping hack, allowing for a midair fight. Attacks, however, such as Kirby's Final Cutter, will freeze the game when performed off the edge.
The purpose for DUMMY remains unknown to this day; while other stages, such as TEST and IceTop were most likely used for programming testing, DUMMY has nothing to show what it was used for. DUMMY may have been used for checking if characters showed up right, though it has been speculated that the stage is used for the victory screens, or the background of the debug menu, along with the screen that Mario, Link, Kirby and Pikachu ran across in the first trailer for Super Smash Bros. Brawl before the screen changed to Battlefield. It could also be a scrapped or unfinished world for training (like in Brawl and Wii U), because the CPU stays idle, like a dummy.