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. The glitch can be performed 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.
The box glitch occurs 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.
Visually, a trapped character appears to be on the floor normally, but from a 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, as throws temporarily disable collisions and allow the character to escape the "box."
Instructions (Jungle Japes)
There is an elevated part of a ceiling below the main platform of the level. This elevated ceiling is surrounded by inward facing walls under the main platform, which are formed by the wooden logs that point toward the camera. 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 hit the floor just beside the wall underneath the floor. If successful, the victim will be trapped in a "box" between the walls, and will be unable to move up or down. The boundaries of the "box" are the normal walls, the floor, and the ceiling. The victim can bounce and tech off these boundaries like they can on any other surface.