As you probably know, as you adventure through Super Mario Bros., each level is numbered according to the world you're in: 1-1, 2-1, etc. But did you know there's actually a "Minus World?" That's right, if you know the right trick you can reach a world that's simply numbered -1, which is how it got its nickname among players. There are multiple methods for getting to Minus World, which vary depending on which platform you're using.
The payoff is also quite different between NES and Famicom. On an NES, you'll get a level that looks identical to underwater world 7-2, but the level can't be finished, as the final pipe will simply send you back to the start of the level (until you eventually use up all your lives.)
If you have a Famicom version of the game and exploit this glitch, you'll get a bizarro version of world 1-3, where you swim through the air while dodging floating Princesses, headless Bowsers, and more. Even better, when you complete -1 world on Famicom, you'll go on to -2 world and even -3 world, which is filled with enemies you can stomp for 1000 points each. Once you complete -3 world, you'll return to the game's title screen. Start it up again, and you'll have entered hard mode Super Mario Bros.—with all the goombas replaced by buzzy beetles.