Double Dragon
The series struggles to maintain a consistent art style after the first game. Though fans found this inconsistency is part of their charm, the series' creator, Yoshihisa Kishimoto, had always lamented that to be the case:

"Many different people worked on Double Dragon's graphics over the years, and Technos often outsourced the game design to external companies, so there was no consistency to the branding or the quality," he says. "I personally find it unfortunate, but that's the way Technos handled its titles."
Contributed by DrakeVagabond
In the first Double Dragon game, Billy Lee wears blue and has blonde hair whilst Jimmy Lee wears red and has brown hair. After this, there have been inconsistencies with this between box art and game releases.
Contributed by Kakariko Kid
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There exists a version of the original Double Dragon made specifically for the Zeebo, a niche console sold in Brasil, Mexico, Russia, India and China. Like Double Dragon Advance, it was a remake that was built completely from the ground up as its own separate game. This version included six missions composed of reworked adaptations of missions 1, 2 and 4 from the original Double Dragon, and three completely new missions. Also included were new enemy and boss characters, and a total of 18 unlockable characters for the game's Extra Battle Mode.
Contributed by Mass Distraction
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The green palette swap of Abobo is actually named Mibobo. This is revealed in Double Dragon Advance where the name was used in a cutscene. It is likely the name is a play on the name Abobo and the Japanese word for green, "midori".
Contributed by Mass Distraction