In Adam: Legend of the Blue Marvel, Adam Brashear, the Blue Marvel, was established as being an active superhero since the 60's and 70's.
Agents of Atlas revealed that the Golden Age heroine Venus is, contrary to previous portrayals, not the goddess of the same name who had her own series and joined the Champions (in order to cut the knot of a particular Continuity Snarl). Aphrodite was not amused.
Agents of Atlas reveals the 1950's Marvel Boy, Robert Grayson, was an Uranian Eternal, brainwashed into believing he was Robert Grayson.
Age of Ultron and Original Sin reveals Angela was always part of the Marvel Universe, being Odin's first born, and the half-sister of Thor. The Nine Realms always had a tenth realm, Heven, that had been cut off from the world tree Yggdrasil.
Alias establishes Jessica Jones went to the same high school as Peter Parker, at the same time Peter was attending high school.
In Alpha Flight #12, James Hudson appears to die. He is resurrected in Alpha Flight #25, where it is revealed James Hudson was transported to Ganymede and is saved by the Q`wrrlln. In issue #26, this is shown to be a ruse made up by the android Delphine Courtney; John Byrne intended to point out the ridiculousness of superhero resurrection stories. Issues #87-90 reveal James Hudson really was transported there, and the events on Ganymede were no longer just a story.
During Bill Mantlo's run, Puck was revealed to have originally been a tall human whose height was reduced by the curse of the Black Raazer, so he was cured and remained active in his "normal" height for a while, and Northstar was revealed to be half fairy due to having a parent from Alfheim. Both of these details were eventually reversed and ignored. {Well, Northstar's origin turned out to be a lie cooked up by Loki, while the Master of the World restored Puck to his shorter yet youthful self. Then these revelations were never brought up again.)
Battle Scars establishes Nick Fury, Jr. is Nick Fury's son, and there is an Earth-616 Phil Coulson.
Black Panther vol. 4 retcons Klaw's origin, how T'Chaka was killed by Klaw, how T'Chaka fought Captain America, and how T'Challa and Storm first met each other.
Avengers #4 establishes Captain America and Bucky went missing in action when a plane exploded in mid-air, but in the earlier Captain America Comics, they are still alive and continue having adventures past the date of the explosion. In What If? #4, taking place in regular continuity, it is explained the Captain America in those stories was William Nasland, the Spirit of '76, taking on the role of Captain America. Jeff Mace took on the role of Captain America, following the death of Nasland. Fred Davis took on the role of Bucky during both Nasland and Mace's career as Captain America. Nasland and Davis' post-war adventures, as part of the All-Winners Squad, was a continuation of the Invaders and the Liberty Legion.
Captain America #153-156 introduced the "Commie-Smasher" Cap of the 1950s, William Burnsidenote who wasn't named as such until issue #602, during the Ed Brubaker run decades later, and his Bucky, Jack Monroe, who'd taken up the mantle some time after Mace and Davis had retired. Originally, Burnside was presented as the legitimate Steve Rogers and Monroe as the legitimate Bucky Barnes, but these stories were also rendered incompatible with canon by Avengers #4; they were thus established as separate characters in this arc, recanonizing the 50s stories while also heavily deconstructing them and their political message, as Steve Englehart regarded the character he was turning into Burnside as a fascistic Designated Hero. Burnside had discovered a Nazi copy of the Super Soldier Serum, and had himself surgically altered to resemble Steve Rogers, going so far as to legally changing his name to "become" Rogers. However, his imperfect serum drove him and Jack mad, causing the government to put them on ice until such a time as they could be cured. However, a far right-leaning low-level employee let them loose, where they fought the original Cap, The Falcon, and Agent 13. Burnside was later used by Doctor Faustus as the Grand Director of his neo-Nazi organization, the National Front, while Monroe was saved by S.H.I.E.L.D. and fixed up enough to become a hero in his own right, taking up the identity Nomad.
Captain America #218-220 explains Captain America did not end up immediately frozen in the Arctic Circle after falling into the English Channel, but first encountered General Dekker, and was then placed in suspended animation and frozen in the Arctic Circle.
Captain America: Sentinel of Liberty #6 reveals there was a Revolutionary War hero, Steven Rogers, who could be considered the first Captain America.
Captain America vol 4 suggests Captain America being frozen in the Arctic Circle was part of a plot by the U.S. Government to interfere with Captain America preventing the atomic bombings of Hiroshima and Nagasaki. This story has not been addressed since, and is probably Canon Discontinuity.
Captain America vol 5 establishes Bucky survived the plane explosion in the flashback in Avengers #4, and went on to become the Winter Soldier.
Dr. Erskine was originally named Prof. Reinstein. The name discrepancy was later reconciled with "Reinstein" being a code name used for the chief scientist of Project: Rebirth, which was passed on from Erksine after his death to the scientist who continued the project as shown in Truth: Red, White, and Black that produced Isaiah Bradley, the original "black Captain America".
Captain America's costume was even subject to a retcon! In his very first appearance in Captain America Comics #1, the red and white stripes at his waistline wrapped around his back, but from issue #2 onward through the short-lived "Commie-Smasher" revival in the 50s, the stripes were only present on the front of his costume, leaving his back solid blue save for a white star. When Cap was revived in Avengers #4, the stripes wrapped around to his back again, and The Invaders (and other Cap stories set in WWII) would show that his costume was always this way, while Burnside's Cap uniform from the 50s had the solid blue back.
A pretty minor one, all things considered, but Frank Miller retconned what age Daredevil was when his father was killed. Originally, he was already in college. In Frank Miller's miniseries, Daredevil: The Man Without Fear, Matt is younger, and is instead in 12th grade. So instead of his father pressuring him to be important and Matt studying and then enrolling in law right before his father died, Matt was pressured to study and picked law... but didn't necessarily have to follow through, since he had already acted as a vigilante at the time and his father was dead.
Deadpool was retroactively added to the Weapon X program; he is not present in Weapon X stories published before his first appearance.
In issue #34 of his first ongoing series, Deadpool's Arch-Enemy T-Ray reveals that Deadpool is not actually Wade Wilson, but stole the identity from the man who would become T-Ray. T-Ray claims the "real" Wade was a teacher, unlike the monstrous mercenary "Jack" who became Deadpool. This clashed with Deadpool's previous Weapon X origin written by the same writer.
A different writer in same series retconned the retcon with a ham-handed explanation that T-Ray was tricking Deadpool to mess with his head; the Weapon X story and the name Wade Wilson became his origin again. Most fans didn't even remember this mess until T-Ray's version was restated as false in Cable & Deadpool, written by Deadpool co-creator Fabian Nicieza.
While traveling backward in time in the 1973 Sise-Neg storyline, Doctor Strange paused in Ancient Egypt for a couple of panels to cast a spell that briefly changed the Thing back to human Ben Grimm, allowing him to escape from shackles designed for the Thing's massive wrists, escape, and begin the defeat of Rama-Tut by the Fantastic Four in FF #19. (In the original Lee/Kirby story, it was vaguely described as a result of the hot Egyptian sun).
A particularly controversial example was Steve Engelehart's retcon of The Falcon's origin in Captain America. When the Falcon first appeared, he was a kindly young social worker who became a superhero to help Cap fight off a group of exiled war criminals. Then Englehart came along and decided that before he put on a costume, the Falcon had actually been a violent, drug-dealing pimp who only became a hero because the Red Skull had brainwashed him in order to have a Mole in Cap's confidence. Enter Rick Remender, who retconned that retcon in All-New Captain America by revealing that Sam Wilson was always the social worker, but the Red Skull had put in the violent, drug-dealing pimp past in an attempt to discredit him, hoping that people would be racist enough to believe it.
The character of Peter Quill aka Star-Lord has been subject to a number of different retcons in order to make his origin somewhat comprehensible in the wider Marvel Universe. The original Star-Lord stories took place 20 Minutes into the Future and possibly in an Alternate Universe, which was slightly reinforced in an Inhumans mini-series which established Star-Lord's father as part of the present day Marvel Universe, with the implication that Peter wasn't even born yet. This was thrown out the window during Annihilation, which tried to position Peter as part of the current Marvel Universe while keeping his original 70's origin. Finally, Brian Bendis just said "Screw it" and started over from scratch, establishing an entirely new (and more coherent) origin that contains bits of the various retcons, but firmly established Star-Lord as a present day Marvel character.
The Sin-Cong conflict is a wide-scale example of this. Aside from Captain America, Magneto, the Howling Commandos and certain Golden Age (or retconned to have been active in the golden age) heroes who were not affected by Comic-Book Time, if part of their backstory involved a real-life war, it's probably retroactively been changed to involve Sin-Cong instead. Reed Richards and Ben Grimm helped the war effort during WWII? Nope, that was during Sin-Cong. Professor X served in the Korean War? Sin-Cong again. What war did The Punisher take part in? You guessed it, Sin-Cong.
In The Illuminati, in-universe, the Illuminati were created after the Kree-Skrull war, of the 1970s. In the real world, they were introduced in the New Avengers, and their origin detailed before Civil War (2006). In The Avengers (Jonathan Hickman), Time Runs Out establishes the group's further activities.
The Rampaging Hulk stories were initially far out stories featuring the Hulk. In The Incredible Hulk #269-287, it is revealed the stories were created as techno-art movies by Bereet the Krylorian. Similarly, an unpublished story by Steve Gerber would have retconned the Howard the Duck stories not written by Gerber as art made by the Krylorian Chireep.
Quite a few regarding the first appearance of the Hulk. Firstly, Bruce's Resurrective Immortality was first activiated in the accident that turned him into the Hulk as he was originally actually killed in the gamma bomb explosion. Additionally, it undoes the Canon DiscontinuityPeter David gave to the Devil Hulk by revealing he was real: he's actually the Immortal Hulk and thus, his actual true form resembles a traditional green Hulk. And that the Immortal/Devil Hulk is in fact the Hulk seen in Bruce's first few adventures.
Brian Banner, Bruce's father, is also shown to have feared the existence of offspring from him would break a spell instead of a fear of something wrong with his genetics, and he'd previously dealt with the Green Door.
The Hulk seen in Secret Empire is really a new personality.
Infinity and Inhumanity retcon parts of the prior Silent War mini-series, namely the bits about the Terrigen Mist being fatal to anyone without Inhuman lineage.
A minor one concerning the data books: When The All-New Iron Manual was released, the original Hulkbuster armor was classified as the "Model 14" armor, retconning it from being a modular piece of armor for the "Model 13" Modular Armor to its own set of armor. When the book was rereleased, the Hulkbuster was restored to being the modular piece of armor, which had the side effect of knocking down all armors after the Modular Armor down a model number.
Iron Man Vol 1 #267-268 originally retconned Iron Man's origin from him being in Southeast Asia to test a new weapon, to investigating problems at a new Stark Industries plant in the region, when he's injured by a mine and captured. It also tied his captor, the Communist Vietnamese warlord Wong-Chu, to the Mandarin.
Iron Man Vol 4 further changes Iron Man's origin as Stark fighting a group of Taliban terrorists in Afghanistan, instead of Stark travelling to Vietnam to rescue Ho Yinsen from Wong-Chu. Ho Yinsen dies in Afghanistan, instead of Vietnam, and his wife and son are later killed.
Iron Man Vol 5 reveals Tony Stark's long lost brother, Arno Stark, was born after Howard and Maria Stark agreed to allow their unborn child to undergo genetic modification from a Rigellian Recorder.
International Iron Man reveals Tony Stark's biological parents were two S.H.I.E.L.D. agents, Amanda Armstrong and Jude.
Iron Man: Legacy revealed that Tony Stark's first attempt at a startup company after Obadiah Stane bought out Stark Industries was not Circuits Maximus, but rather Imperio Techworks. He was forced to leave the company due to a conflict with the Pride.
Marvel: The Lost Generation is a twelve issue miniseries built entirely around retcon - specifically, filling in the blank in Marvel history from 1955 to 1961.
In Marvel Team-Up #74, the Silver Samurai acquired a teleportation ring from John Belushi, while other stories claim it was from Chris Farley.
The Marvel Comics Micronauts were inexplicably brought back to life in a 1996 issue of Cable. This was exactly ten years after they sacrificed themselves to create a Genesis effect that completely restored their ruined Homeworld into a new world at a natural state. In Cable, Homeworld is inhabited by Psycho-Man who is using Baron Karza's old body banks to create dog soldiers.
Commander Rann, Mari, and Bug are now the only Micronauts. For licensing reasons, they are now called The Microns; the others having died in war (Marvel no longer has the license to use Acroyear and most of the others. Huntarr's absence is baffling since he was created by Bill Mantlo, the writer of the first Micronauts series).
Homeworld is once again an overpopulated technometropolis and the Microns are freedom fighters. Homeworld is again under the iron fist of someone who probably has to remain unnamed due to licensing restrictions.
In a never released story (again due to licensing), Baron Karza and Thanos have a fight which merges all of the Microverses into one ,so now Sub-Atomica and Jarella's Homeword are now in the same dimension.
Rann and Mari's appearance and personality are different in every re-appearance. In Cable, Rann is buff and heroic looking while Mari's look screams butch lesbian. And she seemed to have given up the swords for normal futuristic weapons. Then Rann and Mari are looking like their old selves in Captain Marvel, although they don't do much more than talk (kind of like a typical episode of Star Trek TNG). In Realm of Kings: Son of Hulk Rann now sports a goatee and reading glasses. And unlike in Cable where Mari had about three lines between the two issues, she's back to her usual verbose self but now talks like an average Earth bimbo instead of a Homeworld Princess. And look at the man legs on her.
Bug is now a member of the Guardians of the Galaxy and for some reason, he's normal human sized and not about 3 inches which is normal when Microversians travel to the regular Earth universe. Rann and Mari (who is once again inexplicably letting others call her Marionette) have a robot sidekick named Carl (don't ask) and their latest enemy is Son of Hulk. They journey around the Microverse aboard the Endeavor III (which sports the most insanely stupidest ship design ever seen in print or screen: a giant atom). It looks as if someone really hated the deeper, more cerebral Micronauts: The New Voyages.
At least in the Marvel Universe this kind of things can be easily explained away thanks to their *infinite* number of alternate universes. It is entirely possible that the current Micros are just not the originals and that they're as confused as to whom they're dealing with as the heroes are.
As revealed in the late 1990's anthology comic Marvel Universe, minor 1940s Marvel speedsters Hurricane and Mercury were both Makkari of The Eternals under assumed names. And he was part of a pre-Fantastic Four super team called the Monster Hunters.
In New Excalibur, it is revealed there were eight Black Knights preceding Sir Percy. There was also a World War I Black Knight, and a swashbuckling Black Knight.
In Original Sin, it turns out the real Dum Dum Dugan has been dead since 1966, and all subsequent appearances have been an LMD. And Tony Stark may have inadvertently been involved in the Hulk's creation. The Dum Dum retcon was retconned in New Avengers (2015) as that the real Dum Dum was Only Mostly Dead and that the LMDs he used were being controlled by his mind.
When Patsy Walker became Hellcat, the series Patsy Walker became in-universe fiction written by Patsy's mother.
In Frank's earlier appearances in Spider-Man and Daredevil, Frank goes after people for jaywalking and littering. The Punisher 1986 miniseries retcons these stories as Frank being under the influence of mind-altering drugs.
Frank appears to put down his dog, Max, with a knife at the end of one issue. A later story and letters page claims Max survived, as Frank administered emergency medical procedures.
The origin of Quicksilver and the Scarlet Witch have been retconned a number of times. Originally, they were said to be the children of World War II heroes Whizzer and Miss America, only to be revealed years later that, instead, they were the children of Magneto. Flash forward to AXIS and a spell launched by the inverted Witch to harm those in her bloodline, only to have Magneto mostly unharmed, thus making him not their father. AXIS also had a retcon towards Venom - Eddie Brock's Start of Darkness was caused by him taking and publishing a confession by Emil Gregg, who confessed to being the serial killer Sin Eater. However, Eddie was discredited when Spider-Man captured and unmasked the Sin Eater and revealed that he was a completely different person and that Emil was a pathological liar. Flash forward to AXIS: Carnage to reveal that Emil was indeed the Sin Eater and that his lying habit essentially gave him an out.
Invoked intentionally in Secret Invasion, revealing Spider-Woman in New Avengers was a double agent, Queen Veranke of the Skrull Empire.
The Sentry has been around since the 60's. Don't remember? That's the point.
Spider-Woman: Origin establishes Jessica Drew's powers come from being zapped while in her mother's womb by a laser containing the DNA of different species of spiders, instead of being caused by a spider-blood serum and genetic accelerator. Jessica's father Jonathan worked with HYDRA, Miles Warren and General Wyndham, instead of finding and funding research with uranium and working with Edgar Wyndham. Instead of the HYDRA agent Otto Vermis recruiting Jessica into HYDRA, Otto is retired and much older. Jessica is raised by Bova, appearing as a human, and Jessica learns martial arts from Taskmaster.
Truth: Red, White & Black establishes the super soldier serum was tested on black soldiers before being used on Steve Rogers, becoming a major point in the origin of Isaiah Bradley, Captain America, Josiah al hajj Saddiq, Justice, and Elijah Bradley, Patriot.
The fastest turnaround in retcon history may appear in Uncanny X-Men Annual #2. Serving as both a prequel and installment to Dark Reign, it does away with Namor being presented as a skeevy, smelly, creepy old man by turning the dialogue between him and Emma subtly flirtatious.
What If? #4, in an in-continuity story, reveals the creator of the Human Torch android, Phineas T. Horton, created a second android, Adam-II.
The Young Allies series was established as a war-time comic filled with stereotypical depictions of the team, especially Whitewash Jones, whose real name is Washington Carver Jones, in Young Allies Comics 70th Anniversary Special #1.
