Internal Retcon

"Mitch Connor: You know nothing about your dad, right?

Eric Cartman: I know enough. My mother is a Hermaphrodite, so she is actually my father.

Mitch Connor: You really still believe that garbage? The people in your town sold you that line. Come on, you have to have doubted it all along.

Eric Cartman: How would you know anything about who my family is?

Mitch Connor: Heh! I know all about the swindles and schemes in this deadend town. You got lied to, kid, by the people who are closest to you."

- South Park, "200"

Let's face it. Facts are rather inconvenient things. If people know exactly how some really bad things went down in the story, they might freak out a little bit, realizing that they live in a Crapsack World and are liable to be killed senselessly and pointlessly for ridiculous reasons.

Enter the Internal Retcon. This trope allows the reigning authority figure (typically determined by Might Makes Right) to force everyone to pretend like something else is what really happened, simply because the consequences of the truth coming out are really dangerous. Who it's dangerous for, of course, is quite fungible. And an incompetent authority figure is going to do an equally incompetent job if they're trying to enact this trope for less-than-morally-above-board reasons.

Most popular as a way of ending either a story arc or a story proper. Naturally, spoilers are rather unavoidable in describing this phenomenon as a result.

Unperson is a variety where someone is "retconned away". See Treachery Cover Up for another sub-trope. Deceased Fall Guy Gambit is another form of this.

Comic Books

 * One of these is integral in making the ending of Watchmen work —.
 * This is how the first issue of Sin City concludes.

Film

 * A big part of how Men in Black works is that agents use Neuralyzers to wipe people's memories of alien encounters, replacing these memories with more plausible ones. When first involved in the use of one, Will Smith gets mad at Tommy Lee Jones for making a woman believe that her husband left her instead of getting killed and his body used by an alien, and promptly narrates a different Retcon stating that she kicked him out, that she's happy she kicked him out, and that now she wants to go shopping for nice things.
 * Batman in The Dark Knight willingly takes the blame for Two-Face's crimes in order to preserve Harvey Dent's image (and thus all the criminal convictions he racked up) and to restore hope to the people of Gotham. Commissioner Gordon reluctantly goes along with the plan.
 * Nothing particularly interesting happened in Springwood, Ohio, in 1974. Nothing of great interest happened in the years between 1984 and 2003, either.
 * Scott from Austin Powers was introduced as Doctor Evil's test tube son. The next movie reveals that he was conceived as Frau and a time-travelling Doctor Evil's lovechild and states the test tube was a lie.
 * THEY TOLD YOU NOTHING BUT LIES!

Literature

 * The government in Nineteen Eighty Four engages in this trope liberally. In fact the protagonist Winston Smith works at the RecDep of the Ministry of Truth, who are responsible for "correcting historical records" (ie, editing and censoring anything that disagrees with The Party's current statements.)
 * The most extreme example of ret-conning is during Hate Week, when the orator who is busily lauding their ally Eastasia and condemning their enemy Eurasia switches the name of the two mid speech. As the crowd gradually realises that the celebratory flags and decorations of Eastasia are all around, they tear them down, supposing that saboteurs must have arranged for them to be put up. After all, "We've always been at war with Eastasia!"
 * In Harry Potter, after Voldemort's return at the end of the Fourth book, the Ministry of Magic spends most of the Fifth book declaring that Voldemort has not returned at all and painfully persecuting anyone who dares contradict the official story. This ends badly, since they don't make much of an effort to convince Voldemort himself of this.
 * Probably because they believed their own propaganda.
 * In The Fall of the Kings, earlier in the setting world's history, the kings and their wizards were overthrown and the ruling nobility burned all the works about magic that they could find and made it illegal even to claim that magic was real. This causes some frustration for one of the protagonists, a historian living 200 years later who has trouble finding reliable sources for his research on the wizards. Especially when he proposes a debate to prove that the wizards' magic was real, disregarding the fact that the aforementioned law is still on the books...
 * In the Kinky Friedman novel Armadillos and Old Lace this trope may be the motive for murder.
 * In Stationery Voyagers, Katrina's transformation from Bovinez to Mantalone was this engineered by the Xylien Society, which had to Unperson her old identity (and ink color) in a sort of enforced version of That Gel Pen Is Dead in order to protect the rest of her family.
 * With Rhodney's family all missing and presumed dead, the Xyliens help cover up his public embarrassment by simply saying "he has no history... He simply is."
 * By simply leaving out details about who was where and what they were doing at the time, Astrabolo is able to weave the tale of how Liquidon killed Astriliad into the perfect propaganda about a "beloved leader" who was murdered in cold ink. Astrabolo's troops mobilize quite effectively in favor of genocide against all Whiteouts from there, never bothering to ask any more questions.
 * Resident Amoral Attorney Spike Inkfong frequently resorts to spreading Blatant Lies in an effort to turn the public against the Metallic Voyagers, including spreading a completely fabricated new narrative about why they went missing for several years. This new story literally crumbles when said Metallics save townsfolk from a building right before it collapses on three of the Metallics, presumably killing them. (Those three are soon never mentioned again.)

Live Action TV

 * In the Firefly episode "Jaynestown", the crew lands on a planet whose working class inhabitants worship Jayne under the misguided belief that he "robbed from the rich and he gave to the poor, stood up to the man and gave him what for. After the truth comes out, Jayne remarks that probably nobody who was there to witness it understood what happened and they're probably putting back up the statue of Jayne that Jayne himself tore down.

Web Comics

 * An open-ended example occurs in Order of the Stick where.
 * Ironically in the prequel book it turns out

Western Animation

 * The Simpsons: Armin Tamzarian was legally declared the one and only Seymour Skinner, and nobody was ever to mention it again under penalty of torture.
 * Doesn't stop Lisa from doing a Continuity Nod in a later episode. Lisa decides that, despite various numbered Snowballs that have died, the one she has at the end of the episode should be Snowball II. Skinner finds out, and starts to chastise her about it, and she responds, "I suppose that's for the best, Principal Tamzarian." Skinner backs off quickly.
 * South Park: Remember the Cliff Hanger of the first season, which was resolved by revealing that Eric Cartman's father was his own mother, who was actually Hermaphrodite?

Video Games

 * Fire Emblem: Path of Radiance and Radiant Dawn has you trying to prevent war from racing across the world in order to keep the evil goddess Yune sealed within the Fire Emblem. She wakes up anyway, but you learn that she's chaotic, not evil. Her Knight Templar counterpart is pretty nutty though. The only surviving person who knows what actually happened in the first place is the king of the Dragons, and he's the one who fabricated all sorts of stories and villified the chaos god. Why? To prevent war from breaking out in the first place!