From Sonic Retro
Final Fever was a newly designed level that Christian "The Taxman" Whitehead planned to add to his officially licensed 2011 remake of Sonic CD but ended up cancelling. It was intended to be a boss only for players who had collected all the Time Stones (i.e. encountered in the Good Future only). Time was to flow freely, and Eggman's machine would be almost invincible: the idea was that Sonic could reach warp speed at any opportunity (instead of having to pass a warp post) and must do so in order to "wind back the clock" and thereby damage Eggman. Some parts are running, whereas in some areas the screen is locked.