|Unused Enemy||KT15 found unused sprites in Green Hill Zone. These two animations are unused, however, another animation is used, but it is used in Jungle Zone, not Green Hill. This sprite is only found in the Master System version of the game.|
|Unused Enemy||This Bat sprite appears in Green Hill Zone but isn't used. It was probably meant to be in the caves of Act 2. In the 16-bit version this is a normal enemy in Marble Zone, a level not found in this version. This sprite is only found in the Game Gear version of the game.|
One of magazines shots features the screenshot from possible, earlier build of the original Master System game with the hud spelling "RING" instead of "RINGS".
Unused Marble Zone BGM
Ancient Corp. handled the 8-bit ports of Sonic 1 and were evidently going to credit themselves on the title screen with the text "©SEGA/Ancient 1991". In the end, though, only Sega received credit.
(Source: Sonic Retro)