Fun Fact: The original Super Mario Bros on the NES has a ton of unused levels! They can be used by turning on the NES with SMB inside. With the NES on, remove SMB and insert Tennis into the NES. Lastly, remove Tennis with the NES on, and insert SMB into the NES and press start.
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Oh, Cool! Can This Be Done In An Emulator? (On 3DS)
Probably not like this. You could use hacks/cheats on an emulator to access the levels if they are still on the emulator,
Thank you! have a nice day :D