Three levels from Sonic the Hedgehog 2 were cut from the game; and they're called Wood Zone, Dust Hill Zone & Genocide City Zone.
Three of them were cut later in development because they were too difficult and there was not enough memory to include them. Although, they can still be playable in a split-screen multiplayer mode.
Wood Zone[]
Wood Zone
Wood Zone is the first scrapped level that was cut from Sonic 2 during the development because it was too difficult. It was later recycled as Mushroom Hill Zone in Sonic & Knuckles.
Dust Hill Zone[]
Dust Hill Zone
Dust Hill Zone is the second scrapped level that was cut from Sonic 2 during the development because it was too difficult. It was later recycled as Desert Palace Zone in a competition mode of Sonic 3 and Sandopolis Zone in Sonic & Knuckles.
Genocide City Zone[]
Genocide City Zone
Genocide City Zone is the third and final scrapped level that was cut from Sonic 2 during the development because it was too difficult. Contrary to belief, it was already dark and gloomy. It was later recycled as Chrome Gadget Zone in a competition mode of Sonic 3.