If you were reading video game magazines back in 1992 (especially a little publication called Electronic Gaming Monthly) then you probably saw waves of pictures of the upcoming Sonic the Hedgehog 2 from Sega. Chances are you gawked at the neat pictures of the fancy title screen, the weird crocodile badnik, and Sonic's fancy new running graphic. Then in November 1992 when you got your grubby little mitts on the game, you found all those neat scenes from the magazine weren't in the final game. So what happened?

Back then (and now-a-days, too) video game companies sent preliminary beta versions of their games to magazines. The magazines would play the few sample levels in the game, take some screenshots, and spit out a preview of the game in the magazine. Then the beta version would be sent back to the company (in this case, Sega) to be destroyed. Then when the final game was completed, a copy of that was sent to the magazine for final reviewing. Sonic the Hedgehog 2 followed this path to glory and the pictures seen in magazines in early 1992 were from a beta version of the game. However, instead of returning to Sega the beta cartridge found its way into the hands of Asian pirates who quickly began copying and selling the game as the final, complete version in the pirate game market (which eventually made it easy for the beta version to find its way online to the general public).

The beta version featured a slightly different title screen and included four additional levels not seen in the final version: Genocide City, Dust Hill Zone, Wood Zone, and Hidden Palace Zone.

Genocide City and Death Egg Zone (from the final game) weren't actually in the game - only a title card and level spot for them were. Accessing those levels put Sonic into a empty level where he would fall to his death. The Wood Zone was somewhat playable, but was horribly incomplete. Sonic could only progress a little ways into the forest level, complete with giant trees and branches, before becoming stuck. Dust Hill Zone eventually became the Mystic Cave Zone in the final version.

As for the Hidden Palace Zone, it was basically completed and fully playable. Enemies were present, as were rings and other items that one would find in a Sonic game. Therefore, that's the level that most magazines featured pictures from. However, when the game was released it had been removed because it no longer fit with the game's plot. The deletion of this level was noted by players and many fans became obsessed with trying to find it in the final version of the game. After all, since it was the Hidden Palace Zone perhaps it was a secret level. While traces of the level remain in the game, they are only accessible by a Game Genie and the level is completely glitched and pixeled out.

As for the different plot, it seems that our good friend Tails the Fox was to be the guardian of the Master Emerald, the role that Knuckles the Echidna had in Sonic the Hedgehog 3 and Sonic and Knuckles (and, by extension, Sonic 3 and Knuckles). The Hidden Palace Zone was to be where the Master Emerald was hidden (and you can see it in the beta version. It's the giant jewel blocking a transport tube). When this plot was reassigned to Sonic 3, it is speculated that the Hidden Palace Zone was struck from Sonic 2.

Note that the Hidden Palance Zone in Sonic and Knuckles is a completely different level than the one in Sonic 2 Beta. The only similarity they share is that the Master Emerald is contained there.

If you poke around on the Internet for a while you might come across the ROM of Sonic the Hedgehog 2 Beta that is playable on an emulator. It seems that someone out there didn't send the beta version cartridge back to Sega and it fell into the hands of someone with a ROM dumper. If you should sample the game, remember that is it unfinished and not able to be completed. It is for historical comparison only.

http://ssrg.emulationland.org is a mighty resource of Sonic the Hedgehog beta version information. They are devoted to searching the old games to find their hidden and lost secrets. There is a lot of interesting reading material to be found there if you'd like to learn more.
gwm told me of the pirate connection.