The box glitch is a collision detection glitch found in Super Smash Bros. Melee. It appears to generate an invisible box that only affects the affected character, trapping that character until they are grabbed and thrown.
Instructions (Jungle Japes)
The glitch is easiest to perform with Fox against Mario.
Fox and Mario must be on the main platform on the level. Fox then has to 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 invisible. 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. If he attempts to jump, however, he will move as if he had jumped into an ordinary ceiling. The box's boundaries act as any other solid stage structure, thus Mario can bounce and tech off them like any other wall.
The only way to dislodge Mario from the box is to grab him again and throw him.