It's possible to access an additional 256 worlds in the game through a glitch. There are two methods to access them via the Japanese Famicom version:
• Having a copy of both Super Mario Bros. and Tennis, first insert the Mario cartridge and then remove the game at the title screen whilst it's still on. Then insert the Tennis cartridge, and during a match, remove the game and place Super Mario Bros. back in. Reset the console and at the title screen hold down A and press Start.
• The second method involves the Family Basic cartridge and the keyboard peripheral. By inputting a series of code and then swapping the cartridge with Super Mario Bros., reset the console and then hold down A and press Start, which will grant players access to one of the 256 glitch worlds.

The Tennis method can also be done with the western version of Super Mario Bros. but only on the NES-101 model.

Each of the worlds appear to reuse assets from the other levels, resulting in various bugged stages. The Tennis game method is possible presumably due to both it and Super Mario Bros. using the same addresses, but don't place the same check values. Leaving the power on also retains the RAM and the previous check value remains.