* 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]].
* NoExportForYou: The 3D installment for UsefulNotes/PlayStation2 is Japan-only, despite [[NoDubForYou having exclusivly English voices]] [[WhatCouldHaveBeen intended (but cancelled) for exporting the game]].
** Also, from the main series, the Combat School, a mission-based DatingSim.
* PortOverdosed: SNK just loooooooooves re-releasing the original game. It's hard to find a single compilation by them that doesn't include it.
* WhatCouldHaveBeen:
** Early on, Sophia was considered to play the role of Player 2.
** 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 verions 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.
*** 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 POWs originally were blue pallete swaps of Morden's troop. Furthermore, instead of giving items, rescued POWs 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/MetalSlug-MissingInAction/MSAction-Main.htm 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 Ptolomaos, the BigBad of the ''Ptolomaic Attack'' event, and the Stone Turtle is a Rare-quality unit.)