There is a Debug Mode code accessed from the Sound Test in the options menu that is inputted by playing songs in the order 01, 09, 09, 02, 01, 01, 02, 04. This is a reference to the North American release date of the game, which was on 1992/11/24, or November 24th, 1992.

The code to access the Level Select is 19, 65, 09, 17. These numbers reference Sonic developer Yuji Naka's birthday, September 17th, 1965.
Contributed by Vipershark