As any Sonic fan would tell you, Sonic the Hedgehog 2 was a victim to Christmas rushing. Unlike Sonic 06, however, it was still an extremely good game. Three levels were cut from the game; Hidden Palace Zone, Wood Zone, and Genocide City/Cyber City Zone. The first two can be accessed using StH2's Level Select, but will lead in a instant death, due to there being no collision data. However, Hidden Palace Zone and Wood Zone can actually be somewhat played in the Simon Wai Prototype.
The reasons why Dust Hill Zone, Rock Zone, and Winter Zone are excluded is because one is a mockup (Dust Hill Zone), one was cancelled really early in development (Rock Zone), and one was a palette swap of Dust Hill Zone (Winter Zone)
Hidden Palace ZoneEditHidden Palace is the most famous of the three missing zones, and the second most playable. In the Simon Wai Prototype, you can play Hidden Palace. However, only the first act is finished, and even it contains some collision bugs. The concept for Hidden Palace was eventually recycled into the similarly named Sonic & Knuckles zone.
Genocide City/Cyber City ZoneEdit
Genocide City Zone is the second most famous, and the most playable. You see, Genocide City is actually fully playable in the game; it's the third act of Metropolis Zone. The reason how Genocide City got its name is because the developers of the game wanted a dangerous name for a act zone. When realising what Genocide really meant, they changed the name to Cyber City, before recycling it into Metropolis Zone Act 3. Contrary to belief, Genocide City was NOT going to be dark and gloomy, but more like Metropolis Zone.