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, reached through a sequence entered on the title screen. Pressing Up, Down, Left, Right plays a confirmation chime; holding A and pressing Start then opens a hidden Level Select. Entering the debug code (Up, C, Down, C, Left, C, Right, C) activates the overlay, letting players place rings, enemies, and other objects freely. The mode was used extensively by the speedrunning and ROM hacking communities for decades, and highlighted Sega's relatively open approach to leaving development tools in finished software during the Genesis era. (The 19, 65, 9, 17 sound-test code often cited for this trick is actually Sonic the Hedgehog 2's.)