This article pertains to the beta elements from Sonic & Knuckles.
8th Special Stage
Using the Sound Test, select any of the Special Stage options and Sonic or Knuckles will taken to the 1st Special Stage. The end of it however, spawns a Yellow Super Emerald. This is when the Special Stage is marked as clear, but the normal Chaos Emerald amount doesn't rise. The Super Emerald amount will rise, but it has pretty much no effect due to Super Emeralds being unobtainable. This bug will allow you to access a hidden Special Stage and allow for the 8th Emerald to be collected once the other six have, though.
Tails Unplayable
The game only features Sonic and Knuckles, making Tails unavailable to play in Sonic and Knuckles. (Just like Knuckles is unplayable in Sonic the Hedgehog 3)
The True Doomsday Act 2
The real Doomsday Act 2 is inaccessible by normal means unless a glitch is used. The level is empty, except for a few chunks off screen, ad Super Sonic dies after his transformation because he has no rings.
Super Insta-Shield
At one point in development, Super Sonic was supposed to have his own Insta-Shield which was superior to its predecessor. This Insta-Shield would be activated the same way as the Insta-Shield, except once activated, it would release a golden wave of energy instead of white that had a greater range and would last a whole second before having to recharge for about two seconds. This was removed because if it is used, the game will freeze for about three minutes and for some unknown reason, send Sonic to Hidden Palace Zone where the game would subsequently crash. (The move was found in the sprite files)
External Links
Sonic and Knuckles on Sonic Retro.Template:Sonic 3 & Knuckles info