Retcons are what make Neckbeards rage the most. Beyond all else, when fluff gets retconned, especially good fluff, it makes everyone go into fits of anger, and sometimes proves that the people controlling the source material are incompetent, abusive to the universe and (worst of all) make good things die.
The word "retcon" is a portmanteau of "retroactive continuity" . 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 fluff to do so. Or both.
Retcons come in all shapes and sizes, and while not all Retcons are bad, many of them (especially those stemming from Matt Ward) tend to remove vast amounts of depth and character from the setting.
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.
- 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.
- 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.
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, 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)
- 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)
- 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.
- 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.