Hidden Palace Zone
Hidden Palace Zone is a scrapped Level from Sonic 2. It can be found in the beta, along with the music, which is unique to the game. Another unique thing is while playing as Sonic, you can find a Tails Monitor,
Hidden Palace Zone CAN be found in the final version. A Game Genie code can access the zone, but the graphics (not including any items, or Sonic and Tails.) are messed up. ODDLY, Sonic 3 has the level icon. In debug mode of the final game, you can place Oil Ocean Zone`s items. Also, beating the act (possible with debug mode.) will make Sonic go to Oil Ocean Zone.
Icon.
A mockup in a magazine, possibly Gameinformer.
The zone in the final game. Note the Tails monitor.