Sonic Chaos had gone through several changes over its development process. There are several rejected ideas, level designs, and other things that were left on the cutting room floor. The following details several of these things.
Prototype North American Box Art
This box art was shown in magazines, showing Sonic holding Tails' hand as opposed to being on a spring.
Early Game Design
Turquoise Hill Zone
Chaos's reveal showed a very different Turquoise Hill, with darker grass, a different HUD layout and a different sprite for Sonic. The background is also missing details.
Perhaps predictably, the darkness also applies to the trees.
Normally, Tails can't go into Special Stages, but here is is. In the final, the platforms are covered in grass and the big rings use different graphics.