The URL to this site was posted here by someone named "Pokr." It's well-known that Mighty's sprites in Chaotix were edited from Sonic sprites. Well, this site says that Sonic was supposed to be in Chaotix all along. Normally, I'd dismiss this as a hoax, because some elements of what's claimed on the site don't seem very credible (and I doubt the game would say "featuring Knuckles The Echidna" if Sonic was in it, or have the U.S. Sega slogan in a beta version), but that screenshot is rather convincing. I'll leave it up to the reader to decide.
Chaotix Beta Greenboard & Screenshot