Mario Bros Glitch: Negative World (World -1)
There is a glitch in the Mario Bros game on the original Nintendo that allows the player to enter a negative world, World -1, to be exact. The world is just an underwater level from another world that just repeats itself. To get to this world you need to jump while crouching towards the brick wall and this will allow Mario to pass through the wall like they are invisible. The negative world is in the first pipe. If you enter the second pipe it brings you to World 5. Not shown in the video is the third pipe which will bring you to World -1, the same as the first pipe. Pretty cool!