Sonic & Knuckles
Sega had a deadline to meet with Sonic 3 and decided to release the game with with just 6 zones. Eight months later, they released the rest of the game as Sonic & Knuckles. S&K had “lock-on” technology which would allow you to insert Sonic 2 into the top of the game to play through the whole game as Knuckles. If you inserted Sonic 3 into S&K, you got the ability to play as Knuckles as well as play through Sonic 3 with all of the Sonic & Knuckles levels added in to make for a much larger game. You could also insert Sonic 1 in to play the game called “Blue Sphere,” which was a collection special zones much like those found in Sonic3 and Sonic & Knuckles. You could also do this with almost any other Genesis/Mega Drive game, but Sonic 1 was the only one to give you the complete game.