Released on the Sega Genesis in 1994, Sonic & Knuckles is the second part to Sonic 3 and the third part of the "Death Egg Saga". This game had "lock-on technology", allowing the game to interact with past Sonic games. Sonic continues his quest to stop Dr. Robotnik relaunching the Death Egg.
First, enter Mushroom Hill Zone as Sonic and get to the part where you can climb. Press left, left, left, right, right, right, up, up, up on the D-Pad and go to the menu. Hold A and press Start and the level select screen will open. Note that you cannot play stages from Sonic the Hedgehog 3 unless the game is locked on.
There is a debug mode in this game, but you need to lock on to Sonic 3 and Knuckles and do the level select code, now use the code and go to angel island and do the debug code for S3K. Now go to any zone, hit A+Start, and when sonic shows up, press B and place lots of S monitors and spindash into them. The game should glitch and go to Sonic and Knuckles alone with debug mode enabled.