The secret special stage is sometimes called "Eggman's Bathtub".
Some people change what "CD" stands for in the 2011 version, such as "Completely Digital" or "Console Download".
Another more well known one is "Toot-Toot Sonic Warrior" for the game's opening theme.
Keep Circulating the Tapes: The original Sega CD version suffered from this (not the PC port). It was originally going to be included with Sonic Mega Collection, for example, but was left out because Sega was unable to emulate the Sega CD due to missing documentation. They eventually released it as part of Sonic Gems Collection, but this was based on the PC version due to said emulation issues. They eventually decided to hire The Taxman to just recode it from scratch for the 2011 rerelease.
Killer App: If it wasn't one for the Sega CD/Mega CD, it was one of the closest things it had to one.
No Export for You: Not the game itself, but there wasn't a release in America that contained the original Japanese soundtrack until the 2011 release, which contains both that soundtrack and the American one. Rumor has it there were rights issues. Ironically, the reason why the 2011 release almost had Japanese soundtrack only is because Word of Godhas it there are rights issues with the American soundtrack never heard of before. This was eventually righted by the time of the release of the remake, and both soundtracks are now available.
Promoted Fanboy: Christian "The Taxman" Whitehead, who did the downloadable remake. He was responsible for creating the Retro Sonic engine and a number of fangames and is also a member of the Sonic Retro community.
What Could Have Been: The Updated Re-release for digital download services was going to have two brand new levels, "Desert Dazzle" and "Final Fever", on top of all the levels from the original release. Time constraints, as well as a desire to be truer to the original game by not tampering too much with it, resulted in both stages being cut. A glimpse of Desert Dazzle can be seen by entering "32" and "8" into the respective slots on the Sound Test menu (which, incidentally, also unlocks the level select for Tails), while half finished early attempts of the levels can actually be found in the games via hacking.
Whitehead: Yeah, it's colourful since it was intended to be an all Time Stones boss (good future only). Time flows freely, and Eggman's machine is essentially invincible. The idea being Sonic can reach warp speed at any opportunity (instead of getting the warp posts) and must do so to "wind back the clock" to damage the boss. Some parts are running, some areas the screen is locked. I was slightly worried the idea of running to warp might not be self evident (no warp screen, the level and boss would change in real time with some neato effects)