* BTeamSequel: Thanks to SNK's bankruptcy, ''Metal Slug 4'' was farmed out to Noise Factory (the people who composed the music for ''Metal Slug 3'') and the short-lived MEGA Enterprises. ME mostly wound up simply recycling the assets and backgrounds from past games (even the bosses are cobbled together from old sprites), which in combination with the rather poor level design in areas led to complaints that the game feels less like a proper ''Metal Slug'' entry than it does a "tribute" or "ROM hack."
** Metal Slug 5 was originally going to be developed by Noise Factory as well, but SNK took control late in development and revamped it. See TroubledProduction below.
* {{Cancellation}}: ''Metal Slug Wars'' was an [=RTS=] released for iOS around early 2013 and removed from the store within the same year. Most of its assets and music were transferred to ''Metal Slug Defense''.
* FanNickname: Venusians, for the alien race in ''6''. For the longest time they didn't have an official name, and when they did get one it was just "the Invaders", so fans gravitated to a Venus-themed name as a pun based on [[MarsAndVenusGenderContrast their conflict with the Mars people]].
* MilestoneCelebration: ''3D'' and ''Attack'' are dedicated for the 10th and 20th anniversary of the franchise respectively.
* NoDubForYou: The Japanese versions of ''Metal Slug 1'' and ''X'' for the Playstation had their respective Combat School instructors speak in full sentences. While the former port never came out West, the latter one did, yet the instructor is silent throughout. ''3D'' is also this as well, beIng voiced in English. (see below)
* NoExportForYou: The 3D installment for UsefulNotes/PlayStation2 is Japan-only, despite [[NoDubForYou having exclusively English voices]] [[WhatCouldHaveBeen intended (but cancelled) for exporting the game]].
** The sixth game's Combat School is the only version who never came out West. Their school instructors eventually made cameos in ''Metal Slug Defense''.
* PortOverdosed: SNK just loooooooooves re-releasing the original game and the third. It's hard to find a single compilation by them that doesn't include them.
* WhatCouldHaveBeen:
** Early on, Sophia was considered to play the role of Player 2 or at least be of higher importance due to the sheer amount of art for her.
** Early versions of ''Metal Slug 2'''s arcade flyer ([[http://randomhoohaas.flyingomelette.com/msmia/2/co/10.png]] [[http://randomhoohaas.flyingomelette.com/msmia/2/co/15.png]] [[http://randomhoohaas.flyingomelette.com/msmia/2/co/14.png]]) had Fio as a blonde and Eri as a brunette before swapping their hair colors at the [[http://randomhoohaas.flyingomelette.com/msmia/2/co/pa_010.jpg final release]].
** The original version of the first Metal Slug, which was actually completed and location-tested at SNK's Osaka location, was a ''very different'' game. The history of this versions is extensively detailed in [[https://www.youtube.com/watch?v=-0PgNJMP3G4 this French video]], which also contains screenshots and the original background art for it:
*** The entire game was played with the Metal Slug. There was no pedestrian gameplay and Marco and Tarma did not exist at this point.
*** Before Marco and Tarma became a reality, [[http://randomhoohaas.flyingomelette.com/msmia/1/co.html two different characters]] (one looking similar to Fio) were meant to be the main protagonists.
*** The original plot was that General Morden had spent years planning a coup and managed to conquer all of the world’s major cities in his opening salvo. The original player character, Phil John, was an engineer for the Regular Army who was working on the Metal Slug prototype, and took it with him to join [[LaResistance the resistance]]. Player 2 was a gold Metal Slug piloted by one named Michiko Nakajima.
*** The tone was more somber and lacked any humour, being more in line with the developers' work on ''InTheHunt''.
*** The backgrounds were redrawn. For example, the entire first stage, and not just the second half, was set inside a swamp.
*** The game was much shorter: Stage 2 did not have an equivalent in the original version, and levels were shorter and simpler (for example, level 3 did not have the climbing portion or a mid-boss battle). The game was lengthened at SNK's instance, their reasoning being that since the NeoGeo was also sold as an home system, the game should be of reasonable length.
*** The iconic [=POW=]s originally were blue palette swaps of Morden's troop. Furthermore, instead of giving items, rescued [=POW=]s would either climb to the top of the Metal Slug and fire their bazooka, or stay next to it and throw grenades.
** The above video also reveals that rearranged themes from Metal Slug 1 were composed for ''X'' and are still present in the ROM, that an "X" version of Metal Slug 3 was planned, and that the antagonists of Metal Slug 4 (as developed by SNK) would have been fish-people.
*** This is actually corroborated by the ending of Metal Slug 3 where the [[ItWasHisSled Final Mission's backup character]] hurls their handgun in the ocean to symbolize that it is the end... Only for the gun to fall on the head of a devilfish on its way down. Original plans were using that gag screen as the plot hook.
** Many ideas and models were scrapped from the final product, especially in 5. Including a giant turtle-like mecha and the native zapped by the lightning at the beginning as the final boss on a levitating, laser-spitting pillar. See [[http://randomhoohaas.flyingomelette.com/msmia/ this page]] for all the stuff that people have found from the game files. Do note that some of these get their time in the sun in ''[[VideoGame/MetalSlugDefense Attack]]'' (The guy on the Pillar is Ptolomaios, the BigBad of the ''Ptolemaic Attack'' event, and the Stone Turtle is a Rare-quality unit.)
** ''3D'', [[NoDubForYou having exclusively English voices]], was planned for export under the title ''Metal Slug 2006'', also [[MilestoneCelebration to celebrate the franchise's 10th anniversary]], before [[NoExportForYou getting cancelled]] due to ''very''-negative reviews, and SNK decided to release ''[[CompilationReRelease Anthology]]'' instead.
* TroubledProduction: ''Metal Slug 5'', originally being developed by Noise Factory, was almost nearing completion. SNK took over late in development and revamped the game completely, but were forced to rush the overhauled game out prematurely in order to begin development of ''Metal Slug 6'' for the new Atomiswave arcade board.