Sonic The Hedgehog - Cheat Mode
At the Sonic The Hedgehog title screen, press Up, Down, Left, Right. Hold B for level select and sound test.
Sonic The Hedgehog - Debug mode
At the Sonic The Hedgehog title screen, press Up, X, Down, X, Left, X, Right, Up, X, Down, X, Left, X, Right, X, Now hold B and press Start. Press A to enter debug mode.
Sonic and Knuckles - Level select
Start a game with Sonic and run over to the Madmole near the Super Shoes power-up. Directly to the right of the Madmole is a pulley mechanism that takes you to the next level above by pushing Down on it. Instead of pushing down, push the Left, Left, Left, Right, Right, Right, Up, Up, Up. Now press Start to pause the game and press B. You will be sent back to the main screen. When you can select between Sonic and Knuckles, hold B and press Start for a level select.
Sonic 3 - Pick Level
As you hear Sega spoken but BEFORE the main menu appears press: Up, Up, Down, Up, Up, Up. Then at the title screen press Up to get Stage select and Sound Test.
Sonic 3 - Get Extra Stage
After doing the Pick Level code/cheat, go to the Sound test screen and play sounds 01, 03, 05, 07. Now highlight Special Stage 2 and hold B and press START.
Sonic 2 - Be Super Sonic
After you've done the Pick Level code/cheat go to the Sound test screen and play sounds 04, 01, 02, 06. Now collect 50 rings and jump to be Super Sonic.
Sonic 1 Pick Level & Sound Test
First go to the Sonic 1 title screen and press Up, Down, Left, Right. Now hold B to pick a level & get sound test.
Sonic 1 Debug Mode
First go to the Sonic 1 title screen and press Up, X, Down, X, Left, X, Right, Up, X, Down, X, Left, X, Right, X. Now HOLD B and press Start. Now when the game starts press A to go to debug mode.
Sonic 2 Pick Level
First go to the Sonic 2 Sound Test screen and play sounds 19, 65, 09, 17. Now hold X and press START. Now on the title screen hold B and press START.
Sonic 2 Debug Mode
First go to the Sonic 2 Sound Test screen and play sounds 01, 09, 09, 02, 01, 01, 02, 04. Now at the sound test screen hold B and pick a level and Start.
Sonic 3 - Debug Mode
Go to the Stage Select screen and hold button B. Then pick a level and start.
Unlocking hidden games
When you initiate Sonic The Hedgehog 1, 2, 3, Sonic And Knuckles, Sonic 3D blast, Dr. Robotnic's Mean Bean Machine, or Sonic Spinball, the screen states that it is logging the game data. That indicates the game is logging in that you have
initiated that game once. After initiating a game fifty times, it no longer shows that game-logging screen. It stops the logging sequence screen from ever starting a game again and from then on, just starts the Genesis program itself. To unlock a hidden game, you must start a particular Mega Collection game fifty times, continually pressing Z to exit it, then enter into the game repeatedly. Some hidden games do not necessarily require that you start up fifty times, but you eventually need to do so to unlock all the hidden games.
Flicky
Start Dr. Robotnic's Mean Bean Machine before reaching 50 times. Alternately, have a memory card in slot A with a saved game from another Sega game.
Blue Spheres
Start Sonic The Hedgehog and Sonic 3D Blast twenty times each.
Sonic 3 And Knuckles:
Unlocked sometime in the process of starting Sonic 1, Sonic 3, Sonic And Knuckles, and Sonic 3-D Blast fifty times.
Knuckles In Sonic 2
Unlocked in the process of starting Sonic Spinball and Sonic 2 fifty times.
Ristar
Start the following games fifty times: Sonic The Hedgehog, Sonic The Hedgehog 2, Sonic The Hedgehog 3, Sonic And Knuckles, Sonic 2 And Knuckles, Sonic 3 And Knuckles, Sonic 3-D Blast, Sonic Spinball; and start Blue Spheres 30 to 40 times.
Comix Zone (Japanese version)
Select manual mode, then press Z(3), Up(3), Down(3), L, R, Z. A sound will confirm correct code entry.
The Ooze (Japanese version)
Select options mode, then press Up, L, Down, R, Up, Z, Down, L, Up, R, Down, Z. A sound will confirm correct code entry.
Stop Time in Sonic 2
Pause the game and hold A for the rest of the stage to play without time passing. Press X to jump and spin dash while holding A. You will move without time passing, but the enimies will still move. To stop the game just relese A.