What appears to be a Met Generator, judged by the Met-sized mouth and the Met-like features, can be found within the background tiles of Crashman's Stage, which went unused in the final game.
When selecting a Robot Master, if you press and hold the A button, the stars in the background of the intro screen will turn into Pipis. This is just aesthetic as it only changes the intro screen and nothing else.
There is a noticeable difference in music tempo and gameplay speed between the European version of Mega Man 2 and the North American version (the EU version being around 83.3% of the US versions speed.) This is due to conversion issues that occurred during the games localisation from 60Hz (US Standard) to 50Hz (EU Standard). Strangely, Mega Man himself isn't slowed down, making some parts of the game (such as the Metal Man conveyor belts) easier in the EU version of the game.