- An 8-bit version of the game, 'Sonic the Hedgehog (1991/II) (VG)', was released for the Master System and Game Gear. Other than the title, basic story and a couple of similarly-themed levels, the two versions have absolutely nothing in common.
- In the late 1980s/early 1990s, Sega of Europe released a modified version of the Master System and Mega Drive hardware for use in arcades. This multi-cart system was known as Mega-Tech, and the JAMMA version (Mega Drive only) as Mega Play. This machine could host up to eight cartridges at once, meaning the arcade-player could choose any one of eight games to play; both 'Sonic the Hedgehog (1991) (VG)' and 'Sonic the Hedgehog 2 (1992) (VG)' were amongst these games. Other than many notable differences, this version of Sonic 1 was identical to its European Mega Drive counterpart other than the ever-present countdown restricting your play time; you get an additional 3 minutes for each coin you insert. The Mega Play version has a few differences: it's based on the Japanese version of the game, complete with scrolling clouds (see above); the timer counts backward instead of forward, with rather strict limits for each stage (fortunately, if you die and touched a Start Marker, you'll begin at the Start Marker with a reset countdown); any extra lives collected are ineffective (if you want to continue your game, you need a pocket full of quarters); there are only four Zones to play (Green Hill, Spring Yard, Star Light, and Scrap Brain, the lattermost of which is missing its final Act); there are no Special Stages (even if you jump into the giant Ring at the end of an Act, you'll just start the next Act as usual); and finally, there's no credit roll - instead, once you complete the game you are asked to enter your initials on a high score screen.
- All English language versions of the game were changed from the Japanese. A lot of the story data was left out of the English language manuals, No mention is made of South Island or the legend behind the Chaos Emeralds, the enemy names were all changed, Sonic's small animal friends are left nameless and ignored, and, most notoriously, Dr. Eggman was changed to Dr. Ivo Robotnik.
- The version for this game in the Sega Saturn game 'Sonic Jam (1997) (VG)' has the option for players to use Sonic's trademark "spin dash" move in this game. This move was not in the original Genisis/Mega Drive version, as it wasn't introduced until 'Sonic the Hedgehog 2 (1992) (VG)'.
- The Japanese version of the game featured extra visual effects not seen in the Western version. These were added in the period between the two release dates. Changes made were rippling effects in the Labyrinth Zone waters, moving clouds in Green Hill Zone, and the stages being listed in the correct order in the level select menu. Some glitches and bugs were fixed too. This is seen by some as the partial reason why the game was delayed in Japan until after the Western release - so that these changes could be made. This version can be seen in the Sega Saturn game 'Sonic Jam (1997) (VG)'.
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