A demo build for the 3DS version of Sonic Generations was shown at Nintendo Power, which seems identical to the one playable at E3 2011, featuring both Acts of Green Hill and the boss battle against Big Arm. Nintendo Power also included a few screenshots of the first Special Stage.
- The E3 build contained the infamous "uncurling" mechanic carried over from the DS games: Classic Sonic would uncurl after flying off ramps, even if he was rolling prior to being launched. This was fixed in the final version.
- The title screen featured a different design for the "Press START" graphic, and lacked the unique sound effect in the final version.
- The Level selection in the demo was via a simple menu. The top screen showed the background of the Zone and a large artwork of the Sonic selected, while the bottom screen showed the list of Stages. The final game however, uses an adaptation of White Space concept from the console/PC version of the game.
- The demo version did not include the Hint feature.
- The "Start" and "Goal" text were smaller and not in capital letters, except for the "START!" text in Special Stages.
- Classic Sonic has a different shade of blue.
- Both Sonics lack shadows.
Green Hill Act 1
- The structure of the background was completely different and much simpler in the demo.
- Several palm trees were either removed or added between the E3 demo and the final version.
- The Moto Bug at the beginning of the Act was given blue eyes for the final version.
- In the E3 version, the sky uses a simpler texture, there are fewer clouds, and the mountains in the background are fewer and far between.
- In the E3 version, the bridge posts use murkier textures and erroneously float above the ground.
- The water texture is plainer and does not have the ripples in the final version.
- The Results screen showed just a single palm trunk and sunflower. In the final version, the area has tons of foliage and more land.
- Classic Sonic's animation upon getting an A-rank seems a bit different, and he finishes off by looking to his right. In the final version, he looks at the camera.
Green Hill Act 2
- Sonic starts from a standstill in the demo. In the final version, the Act begins with Sonic already running.
- All button prompts are missing.
- Sonic is missing most of his voice clips, especially when he boosts or performs the Homing Attack.
- In the demo, Sonic could Wall Jump without any button prompts. This is not the case in the final version.
- The Boost Gauge does not make any sound when it overcharges. In addition, while the meter is overcharged, it does not show the silhouette of the yellow bar behind it.
- Crumbling totem poles were added to the final version, but lacked in the demo.
- The sea floor that is displayed in the background is a lot lower than in the final version.
- In the demo, the Special Stage's time limit is set at "1:24.11". All Special Stages in the final game have a time limit of 1:30.00. However, this odd number many hint that this timer may have served a different purpose.
- The Boost Gauge is missing the "Y: Boost" graphic. It also lacks the timer that keeps track of how long a single boost lasts.
- The Sonic and Emerald icons on the progress bar does not move in accordance to their object. Instead, they just stay in one place.
- The blue bar and star behind the Sonic outline is missing.
- The blue and yellow Chaos Emerald's positions were swapped in the final version.
- The E3 demo uses a light remix of The Doomsday Zone from Sonic & Knuckles. In the final game, it uses a heavy rock remix of the boss' original theme.
- The boss lacks its health meter in this version.
- When Big Arm smashes the platform, Sonic is stunned by the impact for about a second. In the final version, this only happens if Sonic is very close to the point of impact - otherwise there is no effect.
- (Nintendo 3DS) at The Cutting Floor Room.