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 franchise.
The cutscenes in Super Double Dragon were never fully implemented, but some of the assets that were meant to be used (such as closeups of the Lee brothers and bosses) are still present in the game's data (most of it is compressed and only viewable through save state hacking though).
The PC Engine version has a hidden sprite viewer that shows every character, weapon and animation effect, with frame-by-frame data and adjustable palette. This includes unused enemies from the NES and arcade version that don't appear during regular play (namely the ninja and right-hand men, plus the head swapped version of Bolo), as well as an alternate design for the final boss.
Franchise Ownership Acquisition: The Double Dragon series was originally made by Technos Japan. Technos Japan went defunct following its bankruptcy in 1996 after which a licensing company, Million Co. Ltd, was formed to hold onto the rights to Double Dragon and other Technos franchises. In 2015, Arc System Works bought the franchises from Million Co. Ltd. and has been the developers of Double Dragon games from Double Dragon IV onwards.
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.
The Other Darrin: The Japanese voice cast from the Neo Geo games is completely different from the voice cast used in the Japanese dub from the live-action film.
Port Overdosed: Exceptionally so. Almost every home computer and console in the late 80s/early 90s got a version and the original game has been remade numerous times, even for iOS.
Wayfoward was considering adding the Battletoads and levels based off their series as DLC for the 360 version of Neon (as a nod to the 90's crossover game), but Microsoft turned down the offer.
Super Double Dragon, being an Obvious Beta, is loaded with this. You can read all about it here. According to that article, it was supposed to be a reboot of the series taking place in Los Angeles, and Marian would have been an Action Girl police officer investigating the Shadow Warriors. The Big Bad, Duke, was supposed to be a childhood friend of Billy and Jimmy Lee, and his shadow would have been the Final Boss. The final battle with Duke's shadow would have been a Battle Amongst the Flames. Upon defeating it, Duke would have been given an Alas, Poor Villain moment, and the game would have a Bittersweet Ending, showing Billy and Jimmy mourning Duke at his funeral.
A more direct game adaptation of the movie was in development, with Mortal Kombat-like motion captured sprites. Some of the actors even shot captures for the game. However, it was scrapped due to time and budget constraints.
with the Japanese one