In Act 1 of Green Hill Zone, it is possible to die by going too fast. At a certain point in the stage, the camera may not be able to keep up with you when you dip downhill, causing the game to think that Sonic has fallen into a bottomless pit.

There's footage of a proto-type build of Sonic in the background of the Noah's Arcade ad in Wayne's World. The footage shows UFOs in Marble Zone. These UFOs were removed from the final build of the game.

Sonic was originally planned to have a human girlfriend called Madonna, whose design was directly based on the sexually provocative American pop singer of the same name. In line with this, character designer Naoto Ohshima designed Madonna to be attractive and to chase Sonic around (similar to how Amy Rose acts in later games), which he described as a "male fantasy". Aside from concept art, Madonna's only known appearance was in a 1990 tech demo for the TeraDrive, a Japan-exclusive IBM PC with a Sega Mega Drive integrated into it. There are two separate claims as to why Madonna was removed. Programmer Yuji Naka originally claimed in 1993 that Madonna's planned role was to be a damsel-in-distress, with Sonic needing to save her, and that she was ultimately dropped because the team wanted to move the series in a different, more orthodox direction to set it apart from Mario. However in 2009, Sega of America product manager Madeline Schroeder claimed to be the one who removed her as part of an overall effort to "soften" Sonic's image for younger audiences and foreign countries under the belief that Madonna was too Japan-esque. In 2015, Madonna would be reimagined by comic book writer Ian Flynn and appear in Archie Comics' Sonic the Hedgehog comic book series, having been redesigned to be an agent of G.U.N.'s C.L.I.P. division named "Madonna Garnet".

In issue 260 of Nintendo Power, Yuji Naka revealed that the initial concept for Sonic the Hedgehog's gameplay came from playing level 1-1 of Super Mario Bros. over and over again as fast as he could, since you always had to play the level first regardless of whether you used warp zones or not.
"I always tried to get through the level as fast as I could. And that inspired the initial concept for Sonic the Hedgehog."