The box glitch is a collision detection glitch found in Super Smash Bros. Melee. It traps a character "between" a floor and a ceiling that is below that floor, making all vertical movement impossible. Visually a trapped character appears to be on the floor normally, but from collision detection standpoint that character is also under the ceiling. In many instances, the affected character's movement is restricted further by walls at the edges of the ceiling. The glitch can be broken by throwing the affected character.
The glitch will occur when a character moves down through a floor from above, and collides with a wall besides a ceiling under the floor. Alternatively, the glitch also activates when a character moves up through a ceiling and hits a wall besides a floor, above the ceiling.
The glitch can happen on any stage that has a floor and a ceiling below that floor at a close distance, and a wall facing both and connected to either of them. Original Jungle Japes setup shall be used as an example.
Instructions (Jungle Japes)
There is an elevated part of a ceiling below the main platform of the level. The elevated ceiling is surrounded by inward facing walls, formed by the wooden logs that point toward camera, under the main platform. To perform the box glitch, the thrower has to grab the victim near one of those walls below, and then down throw them such that they hits the floor just beside the wall under the floor. If successful, the victim will be trapped in a "box" between the walls, and becomes unable to move up or down. The boundaries of the "box" are normal walls, floor, and a ceiling. Thus the victim can bounce and tech off them like any other surfaces.
The only way to dislodge the victim from the "box" is to throw them, as throws temporarily disable collisions and allow the victim to pass through the ceiling.