I don't know if this counts exactly, but I had Sonic 3D Blast on the Genesis as a kid. I never really got far in the game, but one day something must've hit the cartridge or something because I got a screen that brought up a level selector so I could go to any level I wanted. I could replicate it too. All I had to was jiggle the cartridge and the game would freeze and take me to this level selector. It was awesome.
EDIT: I actually found an article discussing this exact thing I experienced. https://www.polygon.com/2017/10/3/16409052/sonic-3d-cheat-genesis
Yeah, the programmer did that to pass certification more easily https://www.youtube.com/watch?v=i9bkKw32dGw
[edit] beaten, oh well