Sonic the Hedgehog · Sega Genesis · 1991 · Debug Mode
Effect: Enables a debug mode that lets the player place objects anywhere in the level and toggle Sonic's position freely.
Sonic the Hedgehog's debug mode was a developer tool left in the retail cartridge, accessible through a specific sequence on the title screen and level select menus. Accessing the level select itself required entering a sound test (the notes 19, 65, 9, 17), after which holding A and pressing Start on a stage entry activated the full debug overlay. The mode let players place rings, enemies, and other objects freely and was used extensively by the speedrunning and ROM hacking communities for decades. Its presence highlighted Sega's relatively open approach to leaving development tools in finished software during the Genesis era.