Sonic The Hedgehog 3

Manage to beat a stage at with your time at exactly 9:59 and you'll net yourself an easy 100000 points.

To do this, you must have a copy of Sonic the Hedgehog 2. activate sonic 2's level select (sound test: 19, 65, 9, 17)
Now, without turning the system off, remove the cartidge, and insert the Sonic 3 Cartridge. Reset the Genesis console, keeping the A button held down at all times. On the title screen, press down untill you see sound test.
Success! you now have an easy (but different) way to access the level select and sound test!

If you are afrade of this damaging your games, please, don't try it. I have never had any game be damaged by this trick.

BONUS: If you look at the level Icons, you will see the icons from sonic 2. If you look carefully, you will see the Icon for Hidden Palace Zone, a level that was "removed" from Sonic 2!

Alternately, enable the "Level Select" code and go to "Sound Test". In the level selection menu, play the following songs in order: 1, 2, 4, 5, 6. If done correctly, The Chaos Emerald sound will play. Highlight a level and press Start to begin game play. Obtain 50 rings, jump, and press Jump again while in the air to become Super Sonic.

Enable the "Debug mode" code. Go to the first level in the stage selection screen and keep playing until you get enough points to go to the gumball machine bonus level. After you start, press B to enable debug mode. Press A until you turn into a red spring. Press C to make springs all along the gap in the center. Next, press B again to turn back into Sonic and you will extra power-ups. When finished, use debug mode to transport you under the springs, where you will go back to the level. Note: It is a lot easier if the bouncers on the wall are most all used up.