I'm still working on this section.
Whenever I get this page finished, it will have all the information I've found regarding the "unlimited" special stages that you can play when you put a Sonic 1 cartridge into the top of a Sonic & Knuckles cartridge (this is if you have a Sega Genesis; you can also access these levels using emulators and through the Sonic Collection games for PC and Nintendo GameCube (Sonic Mega Collection).
What I do know so far: If you put certain Genesis games into the top of the Sonic and Knuckles cartridge, a code will be generated based on that game's serial number. The Game Compatibility List indicates which games generate codes, and which ones do not. (If a game doesn't generate a code, the Sonic and Knuckles game will kick in.)
List of level codes
Spam & "spoof" emails
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