Sonic the Hedgehog 2
Sonic the Hedgehog 2
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Sonic Part 2 - Did You Know Gaming? Feat. WeeklyTubeShow
Sonic - Did You Know Gaming? Feat. WeeklyTubeShow
 
In Sonic 2 Remastered for iOS and Android, it's possible to access the original Hidden Palace Zone (from the prototype version of Sonic 2 for the Genesis), referred to as "Proto Palace Zone".

The level can be accessed by unlocking the level select screen and using the Sound Test to input a code, before selecting Hidden Palace Zone. If done correctly the player will be taken to Proto Palace Zone instead.
Contributed by ABOhiccups
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There is an additional post-credits scene in the 2013 re-release that can be seen after collecting all of the emeralds that were missing in the original release. The scene shows the Death Egg crashing into Angel/Floating Island, and cuts to Robotnik angrily jumping on the "END" text, akin to Sonic the Hedgehog. Surrounding him are a number of bushes, through one of which Knuckles' eyes can be seen spying on Robotnik, as a foreshadowing towards Sonic the Hedgehog 3.
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The 2013 iOS version of the game features a green Grounder that can be placed by using the debug mode. This is an homage to Grounder from Adventures of Sonic the Hedgehog.
Contributed by RadSpyro
The 16-bit version of Sonic 2 was released worldwide on Tuesday, November 24, 1992, and was dubbed "Sonic 2sday". This marketing move was not commonly done at this point in time.
Contributed by SonicManEXE
During the game's development, two beta builds were shown off to the public. The first was aired on an episode of the kids' game show Nick Arcade; the other was shown off at a New York toy show and was actually stolen from the event. The ROMs for both the Nick Arcade and Simon Wai prototypes (the latter being named after the person who discovered the ROM image) are available online and feature tons of unused content, including the elusive Hidden Palace Zone.
Contributed by SonicManEXE
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The Genesis cartridge of Sonic the Hedgehog 2 was originally going to come with hologram labels. This was cut because it was too costly.
Contributed by KnowledgeBase
The song that plays during the credits bears a strong resemblance to "Sweet Sweet Sweet" by Dreams Come True, the band credited with writing the first and second Sonic the Hedgehog's soundtrack. Both Sonic the Hedgehog 2 and The Swinging Star, the album on which "Sweet Sweet Sweet" appears, were released in November of 1992.
Contributed by KidDivinegon
Some Samsung phones have a built-in game called "Jump Boy", this game's background music is the Chemical Plant Zone theme from Sonic The Hedgehog 2.
Tails' full name was originally only going to be either "Miles Prower" or "Tails" but not both.

Yasushi Yamaguchi, who originally designed Tails for an internal competition, named him "Miles Prower" as a pun on the term "miles per hour" but the Sonic 2 development team changed his name to "Tails". As an act of rebellion, Yasushi snuck the Miles Prower name into various concept artwork and even in-game tiles. Eventually the developers reached a compromise: "Miles Prower" would be the fox's real name and "tails" would be the nickname he commonly goes by. Sonic 2 also features a cheat code that changes any in-game display of "Tails" into "Miles."
Contributed by MightyKombat
The 2013 iOS/Android phone rerelease of Sonic 2 re-adds the once lost Hidden Palace level. It consists of one act, retains the unused dinosaur and bat Badniks with a few changes, introduces a new jellyfish Badnik and a new Robotnik boss at the end. The 2P Mystic Cave zone music plays in this level.

The entrance to Hidden Palace is found at the bottom of a once-infamous shaft in the Mystic Cave Zone. Originally the shaft had spikes at the bottom that could easily (and slowly) kill the player whether or not they were Super Sonic. The rerelease removes the spikes, thus falling into this hole now takes the player to Hidden Palace.
Contributed by MightyKombat
Genocide City Zone is a scrapped level from Sonic the Hedgehog 2. It was meant to be a single act but ultimately became Metropolis Zone Act 3. According to Sonic 2 level designer Tom Payne, the name "Genocide City" was chosen due to the development team's "not quite complete grasp of the English language" hence the change to Cyber City later on.
Contributed by Funland47
Although his name was changed to Robotnik for the English versions, Dr. Eggman's original Japanese name is still written on the tail fins and bumps on Wing Fortress Zone.
Contributed by gamemaster1991
Tails was originally designed as two different Animals before being turned into a fox. First, he was designed to be a flying squirrel. His design was later used for Ray the Squirrel (who would only appear in SegaSonic Arcade). He was than redesigned as a Tanuki, but this idea was scraped since Mario already had a Tanooki suit that made him able to fly.
Contributed by gamemaster1991
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There was originally a stage called Hidden Palace Zone in the game. It featured an underground cave full of shiny gems. Code for the stage exists on the final version of the cartridge, however it cannot be played through normal means. In an interview, Yuji Naka revealed that the player would be warped to the Hidden Palace Zone when all 7 Chaos Emeralds were collected. There, you would be given the ability to become Super Sonic.

Early versions of the stage can be played in prototypes of the game.
Contributed by Zackee
There is a Debug Mode code accessed from the Sound Test in the options menu that is inputted by playing songs in the order 01, 09, 09, 02, 01, 01, 02, 04. This is a reference to the North American release date of the game, which was on 1992/11/24, or November 24th, 1992.

The code to access the Level Select is 19, 65, 09, 17. These numbers reference Sonic developer Yuji Naka's birthday, September 17th, 1965.
Contributed by Vipershark