Assets from the unfinished level, Genocide City (or Cyber City), from the game Sonic the Hedgehog 2, as well as some art supposedly from an unreleased game called "B-Bomb" are used in the stage "The machine".
The game files include an unused version of the theme for The Machine, during development it shared music with Showdown, and the unused music is from an unfinished prototype.
In the European release, the Options menu music was changed to be much faster when compared to its US & Japanese counterparts.
There's a build date that can be found in the ROM at 0x4C61. It can be displayed in the "Options Menu" by selecting sounds 0, 5, 0, 3, 6, 0 in the "Sound Test".
The Sega Genesis version box art shows Dr. Robotnik as he appeared in the classic Sonic game series. The Sega Game Gear and Sega Master System box arts however show Dr. Robotnik as he appeared in the Adventures of Sonic The Hedgehog cartoon series.
This is the only Sonic game to feature characters originally created for Archie's Sonic comics.
In the initial US version of Sonic Spinball, the game uses the Sonic theme for the title screen, and a remix of the Game Over music for when the player gets a Game Over. They were replaced with new music in subsequent releases because Sega did not actually own the rights to those songs.