Left: Hydrocity as it appeared in the prototype
Right: Hydrocity as it appeared in the final game.
Right: Hydrocity as it appeared in the final game.
- Sonic 3 and Sonic & Knuckles were originally intended to be a single game. However, time constraints, cartridge costs and a deal between Sega of America and McDonald's for a Sonic toy tie-in forced the devs to release the game when it was only half done. To keep the original ambitious vision, Sonic Team decided to split the game in half, saving Knuckles and half the levels for the second part. The Lock-On Technology was developed to allow to combine the games into one as originally intended.
- Flying Battery Zone was originally planned to be in the Sonic 3 half, between Carnival Night and IceCap.note However, it was decided that the zone would be saved for Sonic & Knuckles in order to keep the game length of each part more even.
- Originally, the game was planned to be a 3D polygonal isometric platformer game using the Sega Virtua Processor Chip, the Genesis version of the Super FX chip, that was used in the Genesis port of Virtua Racing. The idea was scrapped due to unsatisfying results and the cost of the SVP chip. The concept of the isometric platformer would be reused later for Sonic 3D Blast.
- An early character document for Knuckles
describes a scrapped ability to create whirlwinds to break down walls. This idea was scrapped, but the ability might have been given to Espio in Knuckles Chaotix. Knuckles' Happy Meal toy also retains this ability. - This game has an item monitor that can only be accessed through debug mode. The item in question is just a blue "S" that grants the character their super form and 50 rings, even if the player did not collect all the Chaos Emeralds. Whether the item was just for bug/balance testing or something more is unknown.
- Similarly, there was a yellow S powerup in the Gumball Machine bonus stage. This, along with the fact the S monitor was present in early version of Hidden Palace Zone, suggests that originally there would have been other means of going super besides the Chaos Emeralds.
- The magazine Sega Magazin makes mention of a "Help item". Sonic would break a monitor and whistle for Tails who would then carry him about akin to a second player. Although not present in the final version, Sonic's whistling animation is still present within the game's code and a whistling sound is heard at the start of The Doomsday Zone.
- In a case of What Could Not Have Been, the Super Emeralds and their powers were not part of the original plan. According to Takashi Iizuka at Sonic Boom 2013, they were only added after the game was split so the player would have an incentive to go through another set of Special Stages.
- A theoretical Knuckles the Echidna in Sonic the Hedgehog was going to be accessible by locking the game to Sonic 1. However, this idea was scrapped, mostly due to palette issues (as all the in-game graphics used Sonic's palette as a base) and bugs involving the gliding mechanic in Scrap Brain Zone. Interviews also state the developers did not consider Knuckles' gameplay enjoyable enough in Sonic 1's more cramped level design. Naturally, in the decades since then, there's been a ROM hack or several that fixes both of these problems. In the mobile remaster of Sonic the Hedgehog, Knuckles is playable, and can be unlocked by collecting all six Chaos Emeralds; in Origins, he's playable from the start.
- Interestingly, if the game is locked on to Sonic the Hedgehog, it gives the player access to all 134,217,728 levels of Blue Sphere, whereas most other cartridges only provide one specific level based on the game.
- The replacement tracks thought to have been made for the Sonic & Knuckles Collection are actually based on the early tracks intended for the Genesis version of Sonic 3 prior to being replaced.
- During development, Sonic Team experimented with different moves for Sonic, such as a ring attack, an air dash (which would be kept as the Fire Shield's ability and one of Hyper Sonic's abilities) and even a move that is extremely similar to Sonic Mania's Drop Dash, before ultimately setting on the Insta-Shield seen in the final game.
- Before Sonic & Knuckles, there were plans to launch an Updated Re-release called Sonic the Hedgehog 3 Limited Edition or Sonic 3 +, which would reportedly have combined Sonic 3 and Sonic & Knuckles into a single cart as originally intended (before it was forcefully split into two). However, for unknown reasons, this never came to fruition. A couple of prototypes did resurface in 2008 however and the idea of having both in one cart would eventually be realized by fans in the form of Sonic 3 Complete.
- According to Yuji Naka in an 1995 interview, Knuckles originally had a harsher and more reptilian/dinosaur-like design early on before they refined it to his current appearance in the final game. His long tail is a leftover from this design.
- At another point, Knuckles would have been another hedgehog, and Tails would have been fond of him due to his resemblance to Sonic. The "hedgehog rival who resembles Sonic" concept would be used years later with Shadow the Hedgehog.
- A piece of level Concept Art briefly shown in a promotional video depicts Super Sonic fighting a dragon-like monster in space, suggesting that this would have been the final boss faced in The Doomsday Zone, before being replaced with Eggman's spaceship and mech. This is further supported by the Japanese Sonic 3 manual mentioning a mural near the Emerald Altar predicting the coming of a legendary dragon that would bring great disaster, and said mural being shown in early level sketches. The player gets to visit the altar in the final Sonic & Knuckles, but there are no references to the dragon's legend, and the mural instead predicts the battle with Eggman's final mech in space. However, the plot point and concept seem to have been picked up years later by Sonic Adventure, which upon revisiting Angel Island's lore showed a similar mural◊ depicting a dragon-like monster destroying a city, and culminated in a final battle as Super Sonic against said monster (revealed to be Chaos' final form).
- A proof of concept for a remaster for mobile devices was made by Christian Whitehead and Headcannon in the same vein as the previous ports, consisting of Angel Island Zone and all of the Special and Bonus Stages. Years later, a full remaster was created within the Retro Engine as part of Sonic Origins. This is not the exact same build as the proof of concept, however, as Headcannon, the sole developer of the port, chose to redevelop the game using the revised Retro Engine created for Sonic Mania. Some details have changed between the proof of concept and the full remaster as a result:
- Tails' sprite when piloting the Tornado was animated in the exact same manner as in Sonic 2, as opposed to remaining static like in the original game.
- The scene where Knuckles gets ambushed by Eggrobo while relaxing actually took place in Angel Island Zone, as opposed to Mushroom Hill Zone.
- Eggman's new laughing sprites were implemented, but they looked different from the sprites used in the full remaster.
- Following the publishing of the proof of concept video, Stealth wrote a post-mortem discussing the technical aspects of the demo, as well as some ideas he would like to implement for the full game. While a few of his ideas were realized with Origins, some were not:
- There would be alternate modes based on Sonic 3 and Sonic & Knuckles alone, as well as a fourth mode with Flying Battery placed between Carnival Night Zone and Icecap Zone as originally intended.
- The Competition Zones would be expanded into full-featured, single-act zones for the main game, in a similar manner to Hidden Palace Zone in the Sonic 2 remaster.
- Originally, Knuckles' alternate paths would all have had him traverse the levels through hidden underground temples. He also was apparently going to have exclusive zones of his own in place of Flying Battery Zone (still the 5th zone by this point) and Lava Reef Zone.
- Hidden Palace Zone was seemingly going to be part of Lava Reef Zone rather than its own zone, explaining why it shares its music with the latter's second act and is labelled as "Lava Reef 4" in the level select. There were also going to be tiny critters around the Emerald Altar, possibly Chao prototypes.
- The ending cutscene of Launch Base Zone is supposed to show the Death Egg crashing into the volcano and Eggman flying off into the background to return to it. When Sonic 3 was rushed to market, the Eggman animation was Dummied Out, and the Death Egg instead explodes in a white flash. When Sonic & Knuckles was released, its title screen showed the Death Egg crashing in the background behind Sonic and the SEGA logo, but the full lock-on game just had the Death Egg fall offscreen after Launch Base Zone, with Eggman still not appearing. It wouldn't take until Sonic Origins to have the transition restored to its original intent.
- Originally, Sonic's toy from the Happy Meal tie-in was to have been able to fold and roll similar to him doing a spin attack in the games. This concept never went into manufacturing for unknown reasons, but concept art of it has been leaked online. The final product was a figure of Sonic that could launch with the push of a button.
- Another piece of concept art for Sonic's Happy Meal toy reveals that the toy would have been called "Let's Do Launch" Sonic. This art features a bed of white smoke rather than the orange fire used in the final product.
