The Avengers: When Anthony "Tony" Masters/Taskmaster first appears, he explains that he has photographic reflexes and can mimic the movements of anyone he watches. He's had this ability since childhood, and spent his life using it for personal gain. Taskmaster (vol.2) revealed this story was fake, as Tony was actually a S.H.I.E.L.D. agent, as was his wife Mercedes Masters. A dying scientist gave Tony a syringe of an experimental Super-Soldier Serum, developed by the Nazis, which he used to enhance his abilities. After using it, the skills and abilities he gained by watching others began to overwrite his own memories. He forgot his wife, his history with S.H.I.E.L.D. and even his own identity. Mercedes crafted an elaborate set-up to help control and guide the amnesiac "Taskmaster" - becoming his handler, gathering intelligence and feeding it back to S.H.I.E.L.D. Taskmaster became the ultimate double agent in the super-villain underground — one who didn't even know what he was.
In The Vision, it is retconned that Vision's half-brother, Victor Mancha, developed a vibranium addiction almost immediately after joining the Runaways, as a way of coping with the pain that comes with being an untrained teen superhero.
During the 50s, Captain America went through an Audience-Alienating Era wherein he beat up civil rights protestors for promoting communism, which was quietly retconned out when two of his original creatives from the 40s brought him back for The Avengers. A 1970s comic established this Cap as an impostor, thereby returning them into continuity, and also made him brainwashed and insane. It also deconstructed the stories where he went against civil rights protesters by revealing that the Sanity Slippage caused by the flawed serum had exacerbated existing racism and Black-and-White Insanity and led to him attacking innocent Hispanics and African-Americans for supposedly being communists, implying these stories were a look Through the Eyes of Madness.
When Nadia Van Dyne was first introduced in All-New, All-Different Avengers (then as Nadia Pym), she was seen as a constantly upbeat Genki Girl with very little putting her down. There was also an issue set during Civil War II where Nadia tried to build a way to get the feuding superheroes to see everything and breaking down into tears when it blew up. At first, this was seen as a naive girl trying to make her way through life and her new world outside the Red Room. This all changed in issue #5 of her 2018 series, where it was revealed that she has bipolar disorder (inherited from her father, Hank Pym) and that a lot of her earlier attitude stems from it.
Black Panther originally joined The Avengers after Captain America #100, where Cap asked him to become an Avenger as a personal favor. Decades later, Christopher Priest's Black Panther run revealed that T'Challa only agreed to join so he could spy on the Avengers, a revelation that subsequently created tension with his teammates.
Bucky Barnes, Captain America's Kid Sidekick during The '40s, underwent this when brought back by Ed Brubaker. The original version of his origin was that he was a cheery fanboy of Cap who accidentally discovered his secret identity and thus was recruited as his partner to keep the truth from getting out. Then Bucky died in a plane explosion and after that putting kids in harm's way looked like a less appealing idea for Marvel. Captain America: Winter Soldier then retconned his first origin as propaganda, with the truth being that Bucky was an orphan who grew up on a military base most of his life and when partnered with Steve was essentially a teenage assassin, intended to do the black ops work Captain America couldn't be seen doing. So Bucky went from kid sidekick to Child Soldierand then to Anti-Hero when he was brought back as Winter Soldier.
And long prior to that, there was the retcon that The Falcon was a former pimp and drug dealer who was brainwashed by the Red Skull to be his Mole in the superhero community. The whole thing is so contentious that it's been retconned in and out of continuity several times.
The miniseries Daredevil: Father retconned that the man Matt saved in the accident that blinded him was molestinghis own daughter, meaning Matt unknowingly gave up his sight to save a monster.
Nextwave was insanely violent and nonsensical, utterly hilarious, and completely non-canon. And then in Mighty Avengers (2013), Spectrum finds out that the villains of Nextwave, the Beyond Corporation, are back. She goes off on a quiet, angry rant about the impossible things she faced, being forced to kill, being changed by the Corporation, and nobody ever believing any of it really happened, even thinking it was a joke. Then she flashes into her Nextwave trenchcoat (but in black), hairstyle, and headband
Spectrum: I bet it was funny. From the outside. I bet everybody had a real good laugh. Well. Auntie Monica's not ☠☠☠☠ing laughing.
Planet Hulk was kicked off by the Hulk killing 26 people during a fight with the Thing, leaving The Illuminati no choice but to send him into space. The fight in question took place in Fantastic Four #533-535, and was nowhere near as tragic as the subsequent retcons would establish. For one, Thing's dialogue strongly suggested that nobody had been killed, and the fight actually came to a peaceful resolution after Banner managed to regain control of himself. The story was even filled with humorous moments, such as civilians placing bets on who would win.
In the 2017 iteration of Runaways, it's revealed that while Nico was off having adventures with A-Force, her old team fell apart as they lost custody of Molly and Klara, Victor's aforementioned addiction problems caused him to get blackmailed into a "secret mission" with the Avengers, and Karolina became disillusioned with being a superhero and walked away.
2020's Immortal She-Hulk managed to retcon some additional darkness into She-Hulk's already-dark role in Civil War II. Past depictions showed her battle injury in that event as serious and traumatic, but non-fatal. The later book revises it into a full death and resurrection, confirming that She-Hulk is deeply enmeshed in Immortal Hulk's new Green Door/The One Below All mythology and it turns out Jennifer actually died before becoming She-Hulk.
War of the Realms: Jennifer "She-Hulk" Walters gets focus in Issue #20 of Avengers as it ties into the event. There, even though she still beats herself up over her new persona ruining her reputation as a charming bombshell, she monologues internally about how that same persona of being fun and sexy got her a lot of unwanted objectification from villains and paparazzi alike, which fed a growing resentment she's been repressing until her new savage persona gave her an outlet. This also calls back to the central theme of her cousin's current series, of the freedom that being a Hulk allows.
Ultimate X-Men (2001): The twist that mutants are an artificial race created by Weapon X devalues the entire Xavier/Magneto ideological struggle and reduces the mutant race a whole from the legitimate next step in human evolution to a deluded race of Transhumans with delusions of grandeur.
The Incredible Hercules: Hercules was originally introduced by Stan Lee as a Foil for Thor. Where Thor is noble, honorable and selfless, Hercules is arrogant, crude, and selfish. while on occasion his behavior was portrayed as problematic, or even dangerous, it was still intended to be endearing and fun. Later on, it's revealed that there are several deep psychological reasons he behaves this way: 1) he grew up in an era where Jerkass Gods ruled and that had a profound influence on him, 2) he is subconsciously still in mourning and punishing himself for accidentally killing his own family millennia ago, 3) being on Olympus for millennia not only bored him out of his mind, but on Olympus parties and revelry last for decades or sometimes centuries and that habit is hard to break. The bottom line is, though, that Hercules's past actions as an Idiot Hero, Casanova, and Blood Knight are sometimes painted in darker colors.
It's also explained why Herc, despite being descended from Zeus, does not use the classical Greek name of "Heracles." He wants nothing to do with Hera, who cursed him with a murderous rage that led to the death of his wife and children, which in turn led to the Twelve Labors for atonement.
Felicia Hardy, aka The Black Cat, was initially just a laidback adventuress who became a thief because it was fun (and to help break her father out of prison). In Spider-Man/Black Cat: The Evil That Men Do, it is revealed that she was raped by her boyfriend, Ryan, in college. The experience led to her becoming bitter and angry, and she became the Black Cat with the intended purpose of killing Ryan for revenge. Unfortunately for her, Ryan died in a car accident, denying Felicia her vengeance and leaving her without closure. Feeling that her life had been stolen from her, Felicia turned to a life of theft.
Norman Osborn based the Green Goblin on an imaginary boogeyman figure that haunted him throughout his childhood, especially whenever his abusive father would lock him in the abandoned Osborn estate.
Norman: When the lightning struck, the house would become brightly illuminated. In the echoes of the rumbling thunder, I fancied I could hear the unearthly chittering of a green, goblin-like creature. Perhaps it was the blood rushing in my ears, but I became convinced I could hear the dreadful thing cackling, waiting for its chance to feast on my bones. Every time the lightning struck, I expected the goblin would be standing there over me, ready to pounce. I began to pray for the darkness to last just a little longer each time... and I began to hate the light.
Originally, no real reason was given for why Miles Warren had a thing for jackals. The 1995 Scarlet Spider Unlimited one-shot revealed that he was once an associate of the High Evolutionary, and that one of their creations was a man-jackal that ended up killing Warren's family.
Thunderbolts: Dr. Karla Sofen (Meteorite/Moonstone) first appeared as psychologist and psychiatrist who disliked being dependent on her patients for income so much, that she entered the super-criminal world as an aide to Doctor Faustus. Then in Thunderbolts #25, her backstory revealed that she grew up in the mansion of Hollywood producer Charles Stockbridge, as the child of his butler Karl Sofen. Karla resented the wealthy family her parents served for having the wealth she wanted and was forced to be a playmate to Charles's daughter Deanna Stockbridge. While Deanna saw Karla as a true friend, Karla envied the wealth she had and started to manipulate the naïve Deanna into giving her things, and other times she merely messed with the poor girl's self-esteem for her own amusement. Karla wanted to get away from the mansion and the wealthy family for their effortless superiority. When her father died of a heart attack, Karla got her wish, and her mother, Marion Sofen, left the mansion to live with her parents with Karla. Karla saw her mother get three jobs to support them and made sure to give Karla the best chances she never had by sending her to the best schools. At Karla's graduation, while Marion felt pride for her daughter, Karla felt contempt for her mother. Karla vowed never to be so stupid as to put anyone else needs over her own and went into psychology in college to understand better how to control others. Then it is shown that Karla Sofen is more evil, cruel, and selfish than she let on. First, Thunderbolts #110 revealed that Karla drove eight patients to suicide and was instrumental in the therapeutic hospitalization of six more. Then Ms. Marvel #38 took her contempt for her mother to its ultimate conclusion when it is revealed that she murdered her mother. And while there were hints, she felt guilt for killing her mother, and she killed her mother because she did not want to see her as a supervillain. When she destroys her mother's grave, she shows that she truly will never put the needs of others before her own despite the guilt she feels or hurting the people she loves.
Bobby Drake, Iceman, was the life of the party, a joker, a little immature but fun to be around, and a bit of a ladies' man. Then it was revealed that he was gay, making all the outward joy and flirty-towards-women behavior he'd demonstrated for decades (out of universe) appear to be a mask he was so desperate to keep up even he might have convinced himself it was true. This came out during a storyline where the original teenaged X-Men come to the present day, and Jean Grey thoughtlessly outs him. Then, at the end of that storyline, when they went back to their correct time, that version of Jean Grey wiped that version of Bobby's mind to preserve the timeline—and pushing him so deep into the closet he in fact forgot about the realization and thought he was straight until the moment it was revealed in his future.
X-Men: Deadly Genesis: Deadly Genesis reveals that before he recruited the All-New, All-Different team to rescue the original X-Men from Krakoa, Xavier recruited another, younger team of mutants for the task. These heroes had a month's worth of training telepathically downloaded into them to prepare them for their mission. Unfortunately, this wasn't enough to make the mission go off without a hitch. Petra and Sway were killed in battle and Vulcan and Darwin were thought dead as well. In reality, Darwin transformed into energy and bonded with Vulcan. When Polaris (though mentioned incorrectly in this miniseries as Jean Grey) blasted Krakoa into outer space in Giant-Size X-Men #1, Vulcan was shot into space as well, but survived in a comatose state thanks to Darwin's reactive powers. After the debacle, Cyclops - the only member of the group who was aware of the "first" wave - was deeply distraught, prompting Xavier to decide to mind-wipe him in order to spare him the agony and grief of knowing that his brother died for him, and later working to clear his name by creating an elaborate illusion that Krakoa was sentient.
Young Avengers: When Kate Bishop first appeared she had a wealthy normal family. Her father, Derek, was a normal wealthy businessman, and her mother, Eleanor, was a kind-hearted socialite who spent her time at soup kitchens and charities trying to help the community before she tragically died. Her older sister Susan had a tense but kind-hearted relationship with Kate because she couldn't understand Kate's desire to help people she inherited from her mother. Despite some dysfunction with her mother's death and her sexual assault, which she had months of therapy where she needed to recover, Kate had a normal relationship with her family, and it's clear that she and they loved each other. But then, in a later series, it's revealed that the Bishop family is more dysfunctional and dark than they first appeared. All early appearances and interactions between Kate and her father are exaggerated in a way that lampshades the luxurious life of rich families, but they take a really dark turn once it's revealed that Derek is really a mafia boss. Kate's mother, Eleanor, was not only involved in her husband's criminal enterprise but was still alive and a vampire running a cult. Kate lampshades this when meeting with her sister Susan, wondering if her sister has some dark secret; thankfully, Susan is shown to be a normal woman, and Kate still has a positive relationship with her sister.
In The Avengers, Tony's flight through the Chitauri wormhole at the climax of the movie is just a good old fashioned fist-pumping action climax, and his resultant brush with death (which he obviously survives) is Played for Laughs, with Tony absentmindedly rambling about going to get shawarma immediately upon waking up. But then Iron Man 3 reveals that he actually got PTSD from the experience, and a minor plot point in the film involves him struggling to cope with anxiety attacks following the battle in New York. Furthermore, his brief foray into outer space, as well as seeing the gargantuan Chitauri army there, instills in him the conviction that the biggest threats to Earth are those from alien invaders. His quest to preemptively combat these threats results in him creating Ultron, supporting the Sokovia Accords, and ultimately breaking up the Avengers, thus rendering Earth more vulnerable to the exact dangers he was trying to protect against.
Senator Stern was more of a comedic pain-in-the-ass than an actual threat, and his attempts to confiscate Tony's armor never really panned out. Then in Captain America: The Winter Soldier, we find out Stern is a high-ranking agent of HYDRA, and was likely trying to take Stark's armor so that the organization could mass-produce their own versions.
Similarly, the World Security Council played Commander Contrarian in The Avengers, pushing Fury to use Tesseract weaponry instead of trying to assemble a Ragtag Bunch of Misfits. The councilman leading this charge later returned in Agents of S.H.I.E.L.D. as, like Senator Stern, a leading member of HYDRA, adding context to why he wanted high-powered weaponry so badly.
This is the whole point of the Sokovia Accords plot in Captain America: Civil War. It's shown that the Avengers' heroic deeds in the past movies have caused untold amounts of collateral damage, and the main villain of the movie is a man whose wife and son were crushed to death by falling buildings during the final battle in Avengers: Age of Ultron. To hammer this point home, footage of The Incredible Hulk fighting the Chitauri from The Avengers is shown, and it's revealed that he accidentally knocked debris onto a crowd of screaming civilians. To make it worse, Age of Ultron made it a specific point to show that the Avengers were doing everything they could to keep civilians out of the line of fire, and the implication was that they had essentially succeeded until Civil War showed otherwise.
In the Iron Man movies (especially 2), Tony suffers from unresolved issues due to his parents' death in a car accident when he was young. Captain America: The Winter Soldier puts a darker spin on it by dropping a heavy hint that their deaths were actually a HYDRA assassination, though this information is seen by Steve and Natasha, not Tony. Captain America: Civil War takes it further, as not only is the hint confirmed and revealed to Tony, but it adds the detail that Steve's brainwashed friend Bucky carried out the hit. Tony is furious that Steve had kept this from him for two years and is now protecting his parents' killer.
Guardians of the Galaxy (2014) has Yondu remark that Peter Quill's father was a "jackass" when one of the Ravagers laments they didn't deliver Quill to him. Come Guardians of the Galaxy Vol. 2, Yondu is proven right when Quill meets his biological father Ego the Living Planet, who wants to eliminate all life in the universe by replacing it with extensions of his Celestial self. Ego had even killed hundreds of his own children after they were delivered to him by Yondu and didn't have his Celestial gene. Yondu even dies trying to save Peter, meaning that he truly became Quill's father-figure after he was abducted. It also has the effect of taking the Running Gag of Yondu going soft and making it so he was fiercely protective of Peter and helped him learn to survive on his own.
The running gag in the series of Rocket disliking being called a raccoon or mistaken for other animals has taken on a much darker meaning after Guardians of the Galaxy Vol. 3. The experiments done by the High Evolutionary to create him made it so he wasn't just resentful of his identity, but deliberately repressing the painful memories of his creation.
At the start of Thor, Odin banished Thor and stripped him of his power after Thor nearly started a war. Thor: Ragnarok shows that Odin had to imprison Hela after she grew out of control in her desire for conquest. Meaning now Odin's banishing of Thor was to stop Thor from becoming a monster like Hela, not just trying to teach Thor humility.
In Captain America: Civil War, Tony nicknaming his holographic therapy machine B.A.R.F. is treated as an offhand joke. In Spider-Man: Far From Home, we see that its inventor Quentin Beck was outraged that Stark ridiculed his life's work with such a demeaning nickname. This was the catalyst that led to Beck becoming a supervillain, and endangering thousands of lives, to make himself look like a hero. Though it's obvious that he was already mentally unstable and Tony would have taken him a lot more seriously than he probably imagined.
Which is why this trope also applies in-universe. Beck remembers people laughing at the name, but in Civil War, absolutely nobody laughed. Beck also mentally edits out Tony saying it needs a better name, represented by a jump in the footage.
Live-Action TV
In Agents of S.H.I.E.L.D., even when the relatively light-hearted first season took a turn for the dark about three quarters through the season note In the wake of Captain America: The Winter Soldier and the revelation that S.H.I.E.L.D. had been infiltrated by HYDRA since day one, it still had a few comedic moments. One of these was the Running Gag that it was obvious to everyone around The Clairvoyant that he had gone insane from the alien blood used to treat his failing cybernetic body (well, Played for Drama with Ward and Reina, Played for Laughs with everyone else). Come the second season, which has a much more serious tone than the first, Coulson's biggest fear is that he too might go crazy since he was revived by that same blood, and this is played deadly serious.
Also in Season 1, every time Coulson was asked about Tahiti, the place he recovered after being wounded by Loki in The Avengers (2012), he'd always say "It's a magical place." until even he started noticing it. It's after he goes through a memory machine that he realizes the awful truth: that he truly did die and that he was brought back to life against his will.
Western Animation
Iron Man: The Animated Series: The first season had an episode where Tony pretended he was going to marry Julia Carpenter as part of a plot to fool the Mandarin. At the time, Julia was clearly in on the ruse, and seemed to have no problem with it. In the second season, Julia is instead shown to be quite bitter about having been left at the altar, and her lingering resentment causes tension with Tony in several episodes.
