Sonic Chaos (Sega Master System)
This game has a prototype article
The Game Gear version on your TV. Chaos!
At the title screen, press Up, Up, Down, Down, Right, Left, Right, Left, 1.
At the title screen, press Down, Down, Up, Up, Left, Right, Left, Right, 1.
Changing RAM address 0xD297 to either $06 or $0D will take you to a level that is just a copy of Turquoise Hill. $06 uses Special Stage music, so it was likely intended for another Special Stage. $0D plays no music at all, which is unusual because it plays the boss theme in the Game Gear version. Both levels function normally with the exception of the boss, which causes camera and VRAM issues.
Stored at xC0 in the ROM:
MS SONIC¥CHAOS < Ver1.20 > 1993/08/03 @SEGA /Aspect Co.,Ltd