If a game with a cartridge's memory size larger than 2MB is attached, the console will not run the lock-on game but booting up the standalone Sonic & Knuckles game instead.

The Sonic & Knuckles cartridge usually reads the second half of the attached ROM. As the ROM header is placed in the first half of the attached cartridge, it will not recognize it as a valid game.

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If the player inserts almost any other Mega Drive cartridge into the slot adapter, again a screen with Sonic characters and a scrolling text "No Way" appears.

This time after pressing the A, B and C button at the same time the player gets to play only a single Special Stage without the password ability based on the serial number of the cartridge's ROM header data.

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Attaching Sonic 1 to Sonic & Knuckles unlocks the Blue Sphere mini game, a collection of Special Stages in the same style of those in Sonic 3 & Knuckles.

After starting the console a screen with Sonic characters and a scrolling text "No Way" appears.
While this screen is visible, pressing the A, B and C button at the same time opens the Blue Sphere menu.

Blue Sphere has 134,217,728 levels and a password feature.

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If you attach Sonic 2 to the Sonic & Knuckles lock-on this unlocks "Knuckles the Echidna in Sonic the Hedgehog 2", wherein the player can play the game using Knuckles and his abilities.

Using these special moves, Knuckles can explore areas inaccessible with Sonic or Tails, where secrets such as extra monitors have been added.

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Sonic & Knuckles has a lock-on feature that allows players to open the hatch on top of the cartridge and insert a second one.

When Sonic 3 is inserted, the player can play through both games as one, the originally intended Sonic 3 & Knuckles.
This features several changes to the games, such as slightly altered level layouts and the ability to play through both titles as Knuckles or Tails.

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