Retcons are what makes Neckbeards rage the most. Beyond all else, when fluff gets retconned, especially good fluff, it makes everyone go into fits of anger, proves that the people controlling the source material are incompetent and abusive to the universe, and worst of all makes good things die.
The word "Retcon" is a portmanteau "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 "Retcon" only applies to new fluff which erases the previously standing old fluff; not 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.
Also, "doublethink" from the book '1984'. In this case, an entire society has a history based on retcons.
- SQUATS (Thanks to 6th edition, they are mentioned to exist).
- 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 death 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.