You may have heard of the infamous Minus World in Super Mario Bros, the glitched ninth world that can only be accessed by clipping into the 1-2 warp zone early. Well, what if I told you there were actually 247 more glitched worlds in the game? Pretty cool, right? But I'll warn you. The way to reach them is even more ridiculous than the Minus World.
To give you an idea of what some of these glitched worlds look like, here's World 68-3:
Yes, that's Mario swimming through the sky and a flying headless Bowser. Levels like this are what you can come to expect from this glitch. You'll notice that it doesn't say "World 68-3", but instead "World Blue Dotted Square-3". This is because that dotted square is the 68th texture in the game, and that text box is simply programmed to display the texture corresponding to the world number. So how do you perform this glitch?
To do so, it will require the Famicom console, a Super Mario Bros cartridge (obviously) and a Tennis cartridge. Yes, Tennis. Load up World 1-1 of Super Mario Bros and wait for a few seconds. Carefully remove the cartridge from your system and replace it with Tennis. Once the game has loaded, hit the reset button on your Famicom, then play a rally or two of tennis. Remove the cartridge whenever you are ready, then put the Mario cartridge back in. Reset the console, hold A+Start in the title screen, and voila, you will load into a glitched world.
The amount of time you spend playing Tennis, as well as the match score when you remove the cartridge can all affect which of the 256 worlds you load into, and it's totally possible for you to end up in one of the standard 8 worlds by chance. This works because the RAM location that is updated in the code as Tennis is played happens to coincide with the place where level data is stored in Super Mario Bros. By holding A+Start after performing the cartridge swaps, the RAM will not be reset and the game will attempt to load World 1-1 from the part of code where the Tennis data is now being stored. As a result, it will load a seemingly random combination of geometry, color palettes, enemies spawns and physics as the level.
Shigeru Miyamoto, the creator of Mario is well aware of this glitch. In fact, when word got around about a "World 9" that was discovered by gamers altering the RAM with their computers, Miyamoto claimed to have been so interested in the commotion, that he added a genuine World 9 to Super Mario Bro 2, accessible only by beating the full game without using warp zones. It appears as an above-ground swimming world, reminiscent of the glitched levels from the game's predecessor.
Fun Fact: When you beat any castle level in the game, the game checks to see if your world count is World 8 or higher. If it is, then Princess Peach will appear and you will beat the game and unlock Second Quest. This means that if you get a castle level when performing this cartridge swap glitch, then you will be able to beat the game and unlock Second Quest, since your world count is greater than eight. Funnily enough, the "But our princess is in another castle" text will still appear, unless you are on World 8 specifically. This occurs on World 54-1 for example:
So dust off that old NES and get down to business. You've got 248 worlds left to beat! If you haven't got the hardware needed to play these wacky levels, check out the world record holding Super Mario Bros speedrunner, Kosmic's series where he plays through these glitched levels to experience them second-hand:
Source: Mario Wiki