Sonic the Hedgehog · Sega Genesis · 1991
A hidden debug mode that lets the player fly freely through levels, spawn any in-game object, and view the game's internal ring counter and position data.
Sonic the Hedgehog's debug mode was a development tool left in the final cartridge by the programming team at Sonic Team. Unlike a simple cheat code, it fundamentally restructures the game's interaction model: movement bypasses collision detection, the player can place any sprite object anywhere in the level, and position data is displayed on screen. The fact that it was left accessible to the public suggests it was not considered a secret so much as an oversight, but it rapidly became one of the most famous hidden features in early 16-bit gaming. It also revealed aspects of game construction — such as the density of invisible trigger objects — that demystified how the game worked for a generation of players.
At the title screen, press Up, C, Down, C, Left, C, Right, C (a chime confirms), then hold A and press Start. In a level, press B to switch in and out of debug mode; in debug mode, press A to cycle through objects and C to place them.