Sonic was going to have a human girlfriend named Madonna. This was to copy Mario's girlfriend at the time, Pauline. She was removed by the product manager, Madeline Schroeder. She wanted to remove Madonna as to "soften" Sonic for children and foreign countries, as she thought the concept was too "Japanese". The concept of a human girlfriend was later used for Sonic the Hedgehog (2006)'s Princess Elise.

Sonic was originally going to be called 'Mr. Needlemouse', a literal translation from the Japanese word for hedgehog.

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.

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."