Sonic the Hedgehog Wiki

The first version, released in North America and Europe, lacked some graphical enhancements that were added to the Japanese release a month later, such as clouds that moved independently of the scrolling foreground in Green Hill Zone, two underwater graphical effects in Labyrinth Zone, and more parallax scrolling in all backgrounds of the levels, as well as the title screen. The Japanese release also corrects the level order in the level select screen once activated.

Years later a revised version became available in Sonic Mega Collection. This version contains all of the graphical enhancements and also fixed a bug in which Sonic would instantly die if he "bounced" from one set of spikes to another due to him not gaining temporary invincibility until his feet touched the ground.

The Japanese version wasn't released in North America and Europe until Sonic Compilation. Sonic Jam contains the two versions released in the Mega Drive's lifespan while Sonic's Ultimate Genesis Collection uses the earlier Japanese version.

In addition to this, due to differences between standard PAL and NTSC refresh rates (50 Hz and 60 Hz respectively), the European version of the game plays at about 83% speed of the NTSC version and the music is also slowed down by the same amount. Subsequent PAL Mega Drive games and some of Sonic 1's re-releases (e.g. Sonic Jam) retained the NTSC versions' correct music speed, although the gameplay remained slower.