Retroactive continuity, or retcon for short, is a new piece of information that imposes a different interpretation on previously described events. In regards to the Metal Gear series, a retcon is a situation in which the storyline of a new game explains or changes an event from a previous one, or attaches a new significance to it.
Retcons are typically used in the Metal Gear series to facilitate a dramatic plot shift or account for an inconsistency. Some retcons add information that contradicts the information that was previously established, while others instead fill in missing background details, allowing the accommodation of prequels/sequels or acknowledgement of real world developments. While creating the series, Hideo Kojima was willing to contradict established continuity on occasion, in order to ensure that "new games are as good as they can be."
Metal Gear 2: Solid Snake
Retcons in Metal Gear 2: Solid Snake (MG2) mainly expands upon events that occured in its predecessor Metal Gear (MG). Some information is given outside the main narrative, i.e. the user's manual or optional radio conversations.
The name was originally transliterated Pettrovich (MG).
Metal Gear Solid
The most notable retcon in Metal Gear Solid (MGS1) concerns the resurrection of the presumed-dead Gray Fox from Metal Gear 2, along with the previously unseen relationship between Solid Snake and Big Boss. Background details given in the "Previous Operations" story recaps (Prev. Ops) and user's manual, alter pre-existing information given in the manual for Metal Gear 2, mainly to reflect real world events regarding the end of the Cold War, but also to alter or expand the histories of Snake, Big Boss and FOXHOUND.
Big Boss established FOXHOUND in the 1990s after appreciating the effectiveness of high-tech weaponry during the Gulf War (MGS1 manual).
Metal Gear and Metal Gear 2 re-releases
The re-released versions of Metal Gear and Metal Gear 2 changes the names of several characters in the games. In addition, all the character portraits were given new and/or updated designs by Yoji Shinkawa in the latter to replace the dubiously legal use of digitized photographs of famous figures.
Metal Gear Solid: Portable Ops
The spin-off game Metal Gear Solid: Portable Ops (MPO) expands upon some of the events of its predecessor, Metal Gear Solid 3. It also introduces many changes to the backstory of Frank Jaeger, a.k.a. Gray Fox, contradicting previous accounts from Metal Gear 2.
Jaeger was a child soldier during the Rhodesian Bush War in the 1980s (MGS2 Dark).
Metal Gear Solid 4: Guns of the Patriots
As the intended grand finale of the series, Metal Gear Solid 4 (MGS4) features extensive retcons in its attempt to wrap up the remaining plot threads from all previous games, some of which had deliberately been left open for interpreation.
On a technical level, a large number of changes are made to the layout and contents of the Shadow Moses Island facility, last seen in the original Metal Gear Solid (see Shadow Moses Island#Behind the scenes for more details). For example, there is an entire casting facility to the north of the blast furnace area, accessed via an elevator that was not present in the earlier game.
The only relationship shown between Liquid and Mantis was during their membership of FOXHOUND in the 2000s (MGS1).
The Mammal Pod was still functional after being recovered from Lake Nicaragua. (TPP).
The Mammal Pod was believed destroyed after the AI's self-sacrifice during the Peace Walker Incident (PW).
Big Boss entered a coma in March 1975, only awakening in early 1984 (GZ, TPP).
Big Boss rescued Frank Jaeger from a forced labor camp after the Vietnam War's end (April 1975) (MG2).
Kazuhira Miller loses his right arm and left leg while in Afghanistan (TPP).
Miller was previously depicted with a full set of limbs (MGS1 manual).
Venom Snake, a body double for Big Boss, was killed in Outer Heaven by Solid Snake (TPP).
Big Boss himself was defeated in combat with Snake in Outer Heaven (MG).
Liquid was taken to the UK in 1976 after the abandonment of Les Enfants Terribles (TPP).
Liquid was taken to the UK upon his birth (MGS1 briefing).
Colonel Volgin survived Operation Snake Eater and fell into a coma (TPP).
Volgin was killed by Big Boss during Operation Snake Eater (MGS3, MGS4DB).
Zero was in a persistent vegetative state by 1984, and initiated development of an AI system as early as 1977 (TPP).
The Patriots' AIs were developed following Big Boss's coups d'état in the 1990s, and Zero's mental state had deteriorated due to old age (MGS4).
The layout of Mother Base during the attack scene (GZ) does not match that shown previously (PW).
^Biographical data is given on the official website, which was also included in art books, trading cards, and other supplementary material.
^This retcon was apparently made late in the development of Metal Gear Solid 3, as a leaked voice casting sheet maintained the account that FOXHOUND was founded in the 1990s.
^Male pronouns are used in the English version of Metal Gear Solid 2, while the Japanese version made no mention of the child's gender.
^In reality, this retcon was used as an excuse to explain why the CQC system was retained rather than returning to the previous games' simplistic melee system.
^This retcon removes any known Japanese ancestry on the part of Big Boss.
^Biographical data is given on the offical website, which was also included in art books, trading cards, and other supplementary material.
^It would be practically impossible for an American citizen to serve in the SAS and GSG9: the former would require he be a citizen of the British Commonwealth and a member of one of the British Armed Forces; GSG9 would require he be a German citizen and serve in the German police for at least two years.