In the US version, there is a glitch on Stage 11 (Clinger Winger) where Player 2 is unable to move, and as a result the stage cannot be completed with two players. The Japanese and European versions fixed this. The emulated version from Rare Replay also fixed this.
There are 4 different sets of dialogue that the Black Queen and Professor T. Bird will speak in the ending cutscene. These sets of dialogue are selected at random.
In the US and European versions, a second player can join by pressing Start on the map screen. As the Famicom lacks a Start button on Controller 2, a menu was added to the title screen in the Japanese version that allows you to choose the number of players.
When the game was localized in Japan, the difficulty was toned down considerably, which particularly shines in levels that are notoriously hard in the US/European versions. The Genesis port is based on this localization, so all level changes as well as the other changes above carry into said version.
Connection to Other Media
In a few levels, there are enemies that can steal health by grabbing the energy bars from the health meter directly, as opposed to attacking the player's character. According to the game manual, these enemies are called 'Vaders, "survivors of the original invaders", referencing the game Space Invaders.
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