The box glitch is a collision detection glitch found in Super Smash Bros. Melee. It apparently generates an invisible box that only affects one character, trapping that character until they are grabbed and thrown.
Instructions (Jungle Japes)
For simplicity's sake, we shall assume that the glitch is being performed using Fox and Mario.
Bring Fox and Mario to the main platform on the level. Have Fox repeatedly grab and down throw Mario; Mario should land directly in front of the hut, below the small, elevated platform. After several down-throws, Mario will be trapped in an odd, invisible box.
The box will be completely and totally invisible (unlike characters masked with a Cloaking Device, who are still slightly visible). It will be very slightly taller than Mario, and it will be the same width as the small floating platform. Though Fox can move through the box as though it isn't even there, Mario cannot pass through it. Both characters can touch and damage each other, but Mario cannot be forced out of the box and thus he cannot be KO'd. Mario will not bounce off of the box when smashed; he will simply move along its boundaries. (If he attempts to jump, however, he will move as though he had jumped into an ordinary ceiling.)
The only way to dislodge Mario from the box is to grab and throw him.