First backwards-compatible game cartridge

Gamers were amazed when Sega unveiled Sonic & Knuckles on 18 October 1994 with a special ‘lock-on’ technology – a slot in the cartridge that allowed players to plug in other games and play as Knuckles in earlier games. Although only Sonic the Hedgehog 1, 2 and 3 worked with this feature, it was the first time anything like this had been made commercially available in console gaming history – allowing Knuckles to muscle in on Sonic’s turf even retroactively!