Lock On Technology was used by SEGA during the development of Sonic The Hedgehog 3 and Sonic & Knuckles. The cartridge for Sonic & Knuckles had a slot at the top, where you would put the Sonic 3 cartridge. A special ROM patch would run, combining the two games into one (Sonic 3 and Knuckles). To date, no other video game in history has used this unique method.
When combined with Sonic the Hedgehog 3, it created Sonic 3 & Knuckles. When combined with Sonic the Hedgehog 2, it made Knuckles in Sonic 2, which allowed the player to play as Knuckles The Echidna, who had not been available for play in any game up to Sonic & Knuckles. When combined with Sonic The Hedgehog or Sonic Compilation, the game generated unlimited "Blue Sphere" special stages, the method used to get emeralds in the game. Putting any other Genesis cartridge on top of Sonic & Knuckles produces a single "Blue Sphere" special stage based on the serial number of the cartridge inserted.