Knuckles the Echidna in Sonic the Hedgehog was a planned add-on for Sonic the Hedgehog which allowed you to play as Knuckles by attaching the cartridge to Sonic & Knuckles. The game was only previewed once in volume 44 of SEGA Megazone.
The game was later replaced by Blue Sphere in the final release of Sonic & Knuckles.
Sega Soccer is a port of World Cup Italia '90, released inside compilations in Europe. This version of the game removed the license from FIFA and mentions any mentions of Virgin Mastertronic (the previous publisher).
World Cup Italia '90 is a port of World Championship Soccer which was released in Europe. This port, however, had a license from FIFA to use the 1990 FIFA World Cup as brand.