Sonic the Hedgehog 2/Cheats

Level select
In the options screen, play music 19, 65, 09, 17, and then press. This should return you to the title screen. Hold and press  again and a level select menu should appear.

Receive all Chaos Emeralds
In the level select screen, play music 04, 01, 02, 06. 06 will be the sound heard when you receive a Chaos Emerald instead of what it normally is, if the cheat was done properly. Now you may go to any level. To turn into Super Sonic, collect fifty (50) rings and then jump.

Debug mode
To get Debug Mode, play songs 01, 09, 09, 02, 01, 01, 02, then 04. Hold while selecting a stage in level select mode by pressing  to get Debug Mode.