This glitch/cheat only works on the sonic 3 and knuckles version(I think)
After getting a super emerald reset the console and you should have the emerald you just got and another ring should show up. In the part where knuckles closes the doorway to the giant ring.
This trick works with any game cart except Sonic 2 and Sonic 3. Plug in any other cart and switch the power on. When you get the screen telling you "no way" hold A, B and C and press start. You can now play a bonus game in the form of the 3D emerald stages.
Sonic 1 cart gives multiple bonus stages. All other carts only allow for 1 bonus stage.
Attach a Sonic 3 cartridge to Sonic and knuckles to get Sonic 3 & Knuckles. Then Start a new game and play as any character. Go find a swinging vine and press left, left, left, right, right, right, up, up, up and then pause and press A. After you're back at the main menu go down twice to find Sound select. (MORE LIKE A LEVEL SELECT) Then choose mushroom hill as your starting level. Go to the thing where you pull down something to go up and enter left, left, left, right, right, right, up, up, up again. Press Start and then Press A. Back at the main menu go to Sound select. To activate debug mode in a level hold A and press start when choosing a level to activate debug mode.
A - Switch gravity - Make your character walk on the ceiling.
B - Activate debug - Enter a selection of debug items!
C - (After debug mode is activated) Place debug item - Place debug item once B is pressed
A - (After Active debug) Switch items - Go through the different debug items
B - (After activate debug) Deactivate debug - Get out of debug mode.
C - Jump - The same jump function
It doesn't matter what character you are, on the first level find something thing you can hang on to. Now hit left, left, left, right, right, right, up, up up. You should hear a ring. Now hit start and hit A to go back to the main menu. Now all you have to do is hold A and start when selecting a character to enable level select.