All of these pictures lost in time

 Oh look it's another Sonic pointless trivia

Back in the '90s, there were plans to release the combined Sonic 3 & Knuckles as a single cartridge, internally referred to as Sonic 3C (normal Sonic the Hedgehog 3 was Sonic 3A, and Sonic & Knuckles was Sonic 3B). Judging from articles in magazines at the time, it was going to be called Sonic the Hedgehog 3 Limited Edition or Sonic the Hedgehog 3+. It was meant to be the only version released in Japan, but due to the delay, Japanese gamers just reverse-imported the American version of the games, and the combined version ended up never seeing the light of day.

Knuckles originally had a harsher, more reptilian/dinosaur-like appearance, which his long tail is a holdover from. Another early design had him as another hedgehog, whom Tails would have been fond of due to his resemblance to Sonic. Yet another early design had him as a mole who wore goggles in bright sunlight and used a shovel as a weapon.

Originally, Knuckles's alternate routes through stages in Sonic 3 & Knuckles would have all had him going through underground temples. He was also originally planned to have entire levels unique to him, in place of Flying Battery Zone and Lava Reef Zone.

Knuckles was originally going to have the power to create whirlwinds to break down walls. The team also experimented with giving Sonic unique moves, including a ring attack, an air dash (which was repurposed as the Hinotama Dash and one of Hyper Sonic's moves), and even a move that was very similar to the Drop Dash in Sonic Mania, before they finally settled on the Double Kaiten Attack.

Hidden Palace Zone was originally going to be part of Lava Reef Zone, rather than a level in its own right, which explains why it uses the same BGM as Lava Reef Zone Act 2 and why it's referred to as Lava Reef 4 in the level select.

One concept art for Sonic 3 & Knuckles shows Super Sonic fighting a dragon-like monster in space, which seems to indicate that this was the original idea for the final boss, before it was replaced with Final Weapon. The Japanese manual for Sonic the Hedgehog 3 even mentions a mural near the Emerald Altar that predicts the coming of a legendary dragon that would bring great disaster. In the final game, no trace of the dragon is found, and the mural instead depicts Super Sonic's battle against Final Weapon.

The 4KiDS dub of the anime Sonic X made an error where they mistook Chaos Control for the name of Eggman's base, rather than a specific power of the Chaos Emeralds. In the first episode, when the Chaos Emeralds begin to go haywire and Eggman's base is engulfed in light, Amy asks "What is that?" and Knuckles, in the dub, replies "It used to be Chaos Control." In the Japanese version, he simply says "Chaos Control..." again referring to the power of the Emeralds. A later episode has Eggman in the dub talk about how Sonic invaded, attacked and destroyed Chaos Control, as his base goes by on screen. It wasn't until Shadow debuted and the concept of Chaos Control was formally introduced to the series that they started using the term correctly.

Sonic Adventure 2 also had some translation inconsistencies and dubbing oddities. There was a very common problem of characters in the English dub cutting each other or even themselves off, since the voice acting matched the pace of the original Japanese lines, which were often shorter than the English ones. One example of this is in the battle with Sonic at the end of the Dark Side Story, where Sonic's battle lines get cut off. He's supposed to say "I'll use your technique!" to hint to the player that he's about to use Chaos Control, and then follow up by saying "Chaos Control!" to activate the ability. However, due to the lines being cut off, he ends up saying "I'll use your- Chaos Control!"

Another translation mistake comes in the cutscene before the battle with The Finalhazard. When The Biolizard teleports away, in the English dub, Shadow says "Is that what Chaos Control is?" implying that the monster used a more advanced or perfected form of Chaos Control, or that it teleporting somehow gave Shadow an understanding of the true nature of Chaos Control. In the Japanese line, he merely said "That was Chaos Control". The name Finalhazard was itself a mistranslation - it was supposed to be Final Lizard. The boss subtitles that accompany it also contain an error - they identify it as "The Prototype of the Ultimate Life". Presumably, this was supposed to say "Lifeform", but got cut off.

However, if you think the English translation was bad, the French one was even worse. A notable example was the line describing how the Biolizard had fused with the ARK and was pulling it towards Earth. In the English dub, this line was given as "The prototype is still alive, and he's controlling the space colony as it's falling to earth! He's become one with the space colony, and is determined to keep it on its collision course!" The French dub translated it as "Le prototype est resté en vie et a placé la station sur une course de collision course de collision avec la planète!", which roughly translates as "The prototype has stayed alive and located the station on a collision course collision course with the planet!" And yes, they did repeat "course de collision".