Luigi Ladder

From SmashWiki, the Super Smash Bros. wiki

Luigi Ladder is technique (sometimes referred to as a glitch) in SSBM and SSBB. It involves using 2 Luigis' Up Special Moves constantly against each other, with each Luigi using his Super Jump Punch after the other Luigi gets hit with the attack. They can constantly go up until they reach the blast line. This does not cause a KO because unless sweetspotted, the attack causes no knockback or hitstun.

This is banned in tournament play, despite being impossible without the cooperation of another player, and in Team Battle without Team Attack turned on.