At the title screen press up, down, left, right. You should hear a ring chime. Hold the A button and press Start.
At the title screen press up, C, down, C, left, C, right, C. You should hear a chime. Hold A and press Start. If you have previously done the Level Select cheat, you will enter the level select menu. Simply hold A and press start on the level you wish to play. In debug mode, press A to toggle between be a sprite and Sonic. While a sprite, press B to enumerate amongst the sprites. Press C to place the sprite. Note that Sonic is now invincible against most things. He will not die if he gets hit with no rings. This has an unusual side affect in version 1 of the US release. If he lands on a row of spikes, he will jump to the next row. As a result of the spike glitch, he will jump back to the first one creating an endless cycle.