Luigi's first appearance in a Mario game was actually in the Game & Watch version of Mario Bros. since this version released before the arcade version.
Contributed by Boyobmas
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Within the arcade release's data, an unused graphic of a snake like creature can be found.
Contributed by xianc78
The music that plays when the player begins Phase 1 is Mozart's Eine kleine Nachtmusik, which is also featured in Dance Dance Revolution: Mario Mix and Super Smash Bros. Brawl.
Contributed by GamerBen144
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Luigi's design was partly due to the result of memory limitations. Mario was duplicated to create the second player, Luigi, and unable to produce any additional colors, the color palette of the Shellcreeper was reused resulting in Luigi's iconic green appearance.
Contributed by KnowledgeBase