Sonic the Hedgehog 2 · Sega Genesis · 1992 · Debug Mode
Effect: Unlocks a full level select menu and a debug mode for placing objects in any stage.
Sonic 2's level select and debug cheats were hidden behind the Sound Test in the Options menu: playing 19, 65, 09, 17 — Yuji Naka's birthday (17 September 1965) — unlocks Level Select, and a second sound-test sequence (1, 9, 9, 2, 1, 1, 2, 4, the game's 24 November 1992 US release date) enables debug placement. The level select gave access to the famously unfinished Hidden Palace Zone, cut before release but still partially present in the ROM. Discovering Hidden Palace Zone through this cheat became a major event in the early Sega fan community and launched years of speculation about what the complete zone would have looked like.