The Master System version which was only sold in Europe and Latin America had a different intro and boss music, a larger view of screen, the Under Ground boss was easier (the balls dropped slower and not randomly), and a minor change in Green Hills Zone. Also there are two versions of this one, one has a bug in Gimmick Mountain Zone that was fixed in the other one.
If you "Lock-On" Sonic the Hedgehog 2 with Sonic and Knuckles, you can play Sonic the Hedgehog 2 as Knuckles the Echidna! Play as Knuckles and discover new areas and item locations. Gather all seven Chaos Emeralds and transform in Super Knuckles and defeat Dr. Robotnik. Go Knuckles! Go where no hedgehog has gone before!!
An 8-bit version of the game, Sonic the Hedgehog 2 (1992/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-'80s and early-'90s, Sega Europe released a modified version of the Master System and Mega Drive hardware for use in arcades. The multi-cart system was known as Mega-Tech, and the JAMMA version (Mega Drive only) as Mega Play. The motherboard could host up to eight cartridges at a time, from which players could choose whatever they wanted to play. Among the supported games available for both Mega-Tech and Mega Play were Sonic the Hedgehog (1991) (VG) and Sonic the Hedgehog 2 (1992) (VG). The alterations made to this version of Sonic 2 were basically the same as those made to Sonic 1; see "alternate versions" for Sonic the Hedgehog (1991) (VG).
The game was developed in the US by Sega Technical Institute and released around the world almost simultaneously, so there are no significant differences between international versions. Of the small changes that were made, the biggest is how Tails' name is displayed in the various menus. In the Japanese version, Tails is referenced as "Miles". In the US and European versions, he's listed as "Tails". However, whether it is displayed as "Miles" or as "Tails", it can be changed to the other by inputting a particular code.
The beta version of this game has an alternate opening, as well as several levels that were not completed because they were pulled from the game before completion. One such level is the Hidden Palace Zone, which later appeared in Sonic & Knuckles (1994) (VG) although it was based on a completely different design. The Wood Zone also failed to make it into the final version.