Emily wrote a great analysis of how to access the infamous -1 world in Super Mario Bros. Turns out that there is a whole continuum of worlds beyond the usual eight.
There is a lot of creative code reuse in Mario. This served to keep the game very, very small—as all early NES games were. […] Mario would check out what number was above the pipe in a warp zone and plug that number directly into the world to which it sent you.
By simply manipulating the label above the warp pipe with a hex editor, you can explore all kinds of alternative levels:
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Some strongly resemble existing worlds with little twists (weird gray blocks all over, gray spits from the castles in random places). Some are like world nine, something which should never have seen the light of day and which only barely work (I note that my emulator I used for making screenshots actually froze up). Some are blank and don’t work at all.