Retcons, a portmanteau of "retroactive continuity", are what make Neckbeards rage the most. Where continuity changes in the form of metaplot are hamhanded enough, retcons go the full Orwell in telling readers the whole past was wrong.
In the world of /tg/, retcons usually stem from fluff that the writers and publishers decide isn't necessary or in the flavor of the setting it is being retconned from... or when a new writer decides that certain fluff doesn't fit with his ideas and conception of how things "ought to" be, without considering the universe he is screwing with first. Or when a company wants to make money on new models and decides to wreak havoc on their pre-established setting and their history and timeline to do so. Or both.
Retcon is tolerable when the "newly unearthed" lore has no way of affecting the lore already established. Here, WGR3: Rary the Traitor's excavation of Sulm in the Bright Desert: before then, nobody gave a shit about the Bright Desert, so didn't mind that Anthony Pryor had made said space-filling wasteland (marginally) interesting. (That module had some serious problems; but the retcon wasn't one of them.)
When done wrong a retcon removes depth and character from the setting. That setting's fans will conclude that the people controlling the source material are incompetent, abusive to the universe and (worst of all) make good things die. They have raped the good goddess Mimesis.
Many people tend to use the word "retcon" inappropriately, especially here on this site. The term only applies to new fluff which erases the previously standing old fluff, as opposed to fluff which was conceived out of thin air and erases nothing, but sucks really bad. It also does not mean fluff which is built around old fluff and, in the process, destroys the value of the standing fluff to a point where it no longer means anything of value within the new context. That, my friends, is a Wardcon.
The term "doublethink" from the book '1984' refers to what is effectively retconning on a societal and historical level.
GW and Retcons
GW's stance on canon has, for many years, been not to actually discuss any issue of canon or lore. The universal quote can be thought of as "Everything is canon, just not always true" and some staff have pretty much said EXACTLY this.
This was developed as a view, quite sensibly, in the early days of GW to stop arguments and fights over lore in their stores - especially at a time when WHFB was a mess and Rogue Trader did not know what it was yet and so lore conflicts and what appeared in publications often conflicted or seemed out of context.
Every-time you see a group of neckbeards at each others bushy throats or a web forum descends into chaos, you would serve the community well in reminding them of this.
No-one is right, no-one is wrong - the galaxy is a vast place and full of warp magic; if it can be conceived by the human mind, it's happened somewhere. Just stop bitching at one another and have fun...
Here's just a few of the quotes and who they came from over the years
"With Warhammer and Warhammer 40,000, the notion of canon is a fallacy. [...] Warhammer and Warhammer 40,000 exist as tens of thousands of overlapping realities in the imaginations of games developers, writers, readers and gamers. None of those interpretations is wrong." Gav Thorpe, Lead Designer GW
"It all stems from the assumption that there's a binding contract between author and reader to adhere to some nonexistent subjective construct or 'true' representation of the setting. There is no such contract, and no such objective truth." (See comments section) Andy Hoare, Game Designer GW
"There is no canon. There are several hundred creators all adding to the melting pot of the IP." Aaron Dembski-Bowden, co-author Horus Heresy series
"Here's our standard line: Yes it's all official, but remember that we're reporting back from a time where stories aren't always true, or at least 100% accurate. If it has the 40K logo on it, it exists in the 40K universe. Or it was a legend that may well have happened. Or a rumour that may or may not have any truth behind it." Marc Gascogne, chief editor Black Library
With this in mind, it's common for people to still refer to conflicts as "retcons" but please bear in mind, this is a GW universe and almost NOTHING is ever canon enough to argue about anyway...
- SQUATS (Thanks to 6th edition, they are mentioned to exist and as of 8th edition they have a pair of models for Necromunda).
- Valedictors and Rainbow Warriors being First Founding Chapters.
- Necrons being Terminators IN SPACE instead of Tomb Kings IN SPACE. They have been dubbed the Retcrons for this very reason. Also back in 2nd they were Chaos robots.
- C'tan being superdeath gods of superdeath and Just as planned, and now being shiny space-pokemon (made out of said gods).
- Zoats and those weird Tyranid diplomats, and Tyranids being a race of individuals with leather-strap clothing and mechanical technology...
- The Lone Guardsman being, you know, a guardsman, the great Ollanius Pius. (Apparently Fantasy Flight Games has brought him back. They're awesome like that.)
- A metric ton of goofy Orkish stuff, which, oddly enough, few people missed. Mostly because Ork canon does what it pleases.
- Sisters of Battle not sucking (since they got unique Magic System: Acts of Faith).
- Ultramarines being BRITS IN SPAAAACE.
- Hrud being RATS IN SPAAAACE. They got cooler, though.
- Asdrubael Vect's bitches.
- Space Marines being COPS IN SPAAAACE.
- Jain Zar's origin
- Giving reasons for why Grey Knight would fight non-chaos groups.
- Goff Rockaz.
- Slann, if you think about it.
- With few exceptions, the materials from Battle Technology magazines are considered non-canon.
- Other than that, the developers try very hard not to do major and blatant 180s with the canon, to the point that they try to justify/acknowledge/retool crappy stuff from that clicky game when converting them into the BattleTech proper. For anything else, they rely on the in-universe unreliability of sourcebooks to explain mini-retcons. Ex: The crappy cartoon becoming a crappy propaganda cartoon in-universe with the real events being covered in books and with a few characters migrating to BattleTech proper, most notably Adam Steiner.
- One major retcon came early on due to necessity as the game was made when the USSR hadn't collapsed yet. The retcon was that in the 80's a fictional hardliner inherits Soviet leadership instead of the real life government and this causes the USSR to survive until the 2010's, which also causes NATO to unify into the Western Alliance and after the Second Soviet Civil War's aftermath and a deal with China to reform into the Terran Alliance as the first global government, providing the initial backstory of the setting and the reasons why the colonial planets and the inner planets have longstanding enmity.
- Something that didn't affect the plot, but the artwork and designs: Long story short, there was a legal row over using artwork of licensed designs and this caused BattleTech to pull their designs while stile confirming they existed in lore, making them the Unseen. Eventually they decided "Screw it, we are retconning the appearances." and redesigned them. A summary of the affair can be seen in the Unseen page on the Sarna wiki, Battletech's main wiki.
In Dungeons & Dragons:
- The Spellplague.
- Everything that happened in the Dark Sun novels was eventually retconned away by the 4e setting book and permanently banished through Eberron-style anti-continuity.
In DC Comics:
- Just about everything when they feel like it.
- Just about everything when they're in Alternate Universe mode, which is surprisingly often.
- Just about everything pre-1956.
- Just about everything pre-1986.
- Just about everything pre-2011.
- Just about everything pre-204X, presumably.
- There were some smaller scale Retcon storms in between, but those are the big ones.
In Star Wars:
- Everything - well, not everything, just all material that was not found in the first six movies and Star Wars: The Clone Wars, but only Star Wars: The Clone Wars the cgi animated television show (and the animated movie that kicked off said series), not the other stories from the Dark Horse comics series of the same name featuring the same characters. Although it should be noted that both series came to an unceremonious and sudden end after being acquired by the House of Mouse... not that it keeps them from resurrecting the now-designated "Legends" for newer products.
Notables Rendered Unto Non-Existence:
- Kyle Katarn (and by that extension also Jan Ors, his love interest, as well both his disciples Jaden Korr and Rosh Penin). Notable for being a double retcon, since his opening act - Dark Forces - wasn't canon at release.
- Mara Jade
- Meetra Surik (aka the Jedi Exile)
- Darth Vitiate / Valkorion
- By extension, everyone involved in the KOTOR and SWTOR plotline.
- Darth Talon
- Galen Marek / Starkiller (though it can be argued if The Force Unleashed was entirely canon to begin with, LucasArts explicitly noted gameplay mechanics are not canon even before the Legends continuity was separated, and the TFU story was later given through a comic book)
- Kyp Durron
- Corran Horn
- Wraith Squadron
- Jaina Solo, Jacen Solo, Anakin Solo, Ben Skywalker.
- The Yuuzhan Vong
- The Sith as a Race.
- Korriban - it's now "Moraband." It's More Bad.
Boba Fett surviving the Sarlaac Pit.He's confirmed alive again in The Mandalorian show, and having taken over Jabba's Palace he now sits poised for his spinoff series, The Book of Boba Fett. So, Boba Fett being Mandalore and kicking ass.
- Canderous Ordo
- Nomi Sunrider
- Satele Shan
- Dash Rendar (Dubiously. His ship sorta gets a cameo in the remaster of A New Hope)
- Exar Kun
- Leia and Han's Continuing Marriage.
- Mission Vao
- Pretty much everything even remotely awesome about non-movie Sith barring that one illusion of Darth Bane in TCW.
- Some ancient Sith Lords who make up 99% of the aforementioned awesomeness.
- Luke Skywalker's awesomeness.
- Actually, Luke Skywalker not being a piece of shit who almost murdered his 15 year old desperately help needing nephew for literally nothing but the Dark Side tempting him. Legends Luke forgave his nephew for becoming Caedus even though his wife was murdered by him (said nephew went through so much shit it is arguably more justified than even Anakin's fall). Guy had a genuinuely understanding compassion.
- Cade Skywalker
- Gillad Palleon
- About 75% of Thrawn's awesomeness, the 25% left is him being the only interesting non-Jedi Rebels character.
- The eventual peace between the New Republic and the Empire.
- George Lucas's dignity. (South Park called it. True weird facts about history: that keeps being true.)