This is a glitch that can be performed by using Mr. Game & Watch and Ness. Use Mr. Game & Watch to absorb (using his bucket) three of Ness's fully charged PK Flash attacks. Then, have Ness (or any other character) put up their sheild. Once you unload G&W's bucket onto this player's sheild, G&W will become an unstoppable projectile, and will fly across the screen with blinding speed, instantly KOing any opponent in his path. This can also be done with either Samus (Charge Shot) or Mewtwo (Shadow Ball), but with less effectiveness.
If this is done near a wall, G&W will stop at the wall and ride out the rest of the momentum of his attack. If you jump over the wall before the momentum disappears, G&W will continue over the wall, despite the fact that he came to a complete stop.