The Luigi Ladder is a technique used in Super Smash Bros. Melee (though possible to perform to varying degrees in most games in the series) whereby two Luigis perform their Super Jump Punch on each other with precise timing. Crucially, they must be positioned in such a way that the attacks do not land with their sweetspot, and instead barely connect.
Performing it correctly in this manner results in the Luigis being able to rise up indefinitely, revealing some uncommonly seen inner workings of the game's engine. While it may normally be assumed that rising up too far above the stage would result in a KO since that is what usually happens in regular gameplay (with characters being launched upward), the reality is that a KO above the upper blast line only occurs when an attack causes knockback.
Since the non-sweetspot Super Jump Punch does not actually cause knockback, the Luigis are able to pass the upper blast line and continue rising indefinitely. The technique is banned in tournament play since a team of two Luigis with friendly fire turned on could potentially use it to rise above the stage and become utterly impossible to hit for the rest of the match, giving them an unfair advantage.