From SmashWiki, the Super Smash Bros. wiki
A closeup of Fox
holding a shadow ball from the glitch.
The shadow glitch is a glitch exclusive to version 1.0 of Super Smash Bros. Melee. It was discovered by INF8, Veg, and Simna ibn Sind; it allows the Shadow Balls in Mewtwo's forward throw to be caught as items. This requires three characters: one Mewtwo, one character to catch the shadow ball, and one other character to be thrown.
- The player must have Mewtwo grab and forward throw another player (this works well with Bowser).
- Then, he/she needs to make the third character jump and air dodge into the resulting stream of Shadow Balls and press the Z button to catch one. This will not make the normal item-catching sound effect, so the player must look at the character's hands afterward to determine if the attempt was successful.
- If the Shadow Ball is thrown, it will freeze the game unless it is quickly recaught.