Ascended Meme: "Bimmy", a misspelling of Billy's name in the 2-player intro to Double Dragon III (NES), is reused in DD Neon as a pair of Abobo-sized mutant clones of the Lee Bros. named "Bimmy and Jammy": they also have the Boss Subtitle "Mistranslated Mutants" to make it even more obvious.
The American version of the third arcade game added item shops where the player could purchase new characters, weapons, and new moves by inserting more tokens into the cabinet. In the Japanese version, the non-Lee characters are selectable from the start, weapons are simply found on the ground, and the special moves are usable from the get-go. The only trade-offs are the fact that the Hurricane Kick is harder to pull, and there's no way to increase one's max health and attack speed like in the American version.
Super Double Dragon is based on an earlier build of the game compared to its Japanese counterpart (Return of Double Dragon). Thus it's missing a few moves (such as the ability to catch your boomerangs during its return or change between moves), the Options menu, a few tunes, and the last two areas of the final stage.
The original arcade version began development as a Nekketsu Kouha Kunio-kun sequel. The reason why the characters and settings were changed was done to avoid developing a separate version for the foreign market, as was the case with Renegade.
Rage of the Dragons, which was initially planned as a sequel to the Neo-Geo fighting game, but Evoga could not secure the rights to the Double Dragon IP.
Fan Nickname - The final boss in the first game is nicknamed "Machine Gun Willy", for obvious reasons. This also serves to avoid confusion with this game's run of the mill mook, Williams.
The manual for the first NES game gives out incorrect inputs for the Elbow Punch and Spin Kick, suggesting that these moves were planned to be performed differently at some point. The manual also claims that Abobo likes to throw bombs (actually a mistranslated reference to an "atomic suplex" move he was supposed to have) and spells Rowper's name as "Lopar".
The English manual for the Master System version has the names of Jeff and Willy switched (compare the English manual with the Japanese one). Although the name "Willy" seems more appropriate for a head-swapped evil version of Billy and Jimmy than "Jeff" in retrospect, so it's debatable whether this was a mistake or an intentional change.