Enable Control Mode. After pressing Start to begin a game, hold A until you see Sonic. If you
see a bunch of hex numbers on the top of the screen, you're in Debug Mode.

To start debugging, press B, then you can select any sprite in that level and put it on the screen.
To select a different sprite, press A, and to place it on the screen, press C. Note that sprite
selection is different from Zone to Zone. Also, instead of displaying the time elapsed, it displays
the number of sprites currently on the screen. Control Mode functions work as well.

Note: This only works on the first production releases of Sonic.