She-Hulk: Attorney At Law presents an interesting retcon to her comic book origin. Instead of Bruce Banner willingly offering a blood transfusion to his cousin Jennifer Walters, their blood mixes inadvertently. This makes sense in the context of the MCU, but not all Marvel Comics retcons met with approval from fans. Many proved controversial and still do.
Retcons serve a purpose in comic books, especially serialized ones that have been running for decades. Sometimes, they misstep, as many related to Spider-Man did. Fans still debate the decision to undo his marriage to Mary Jane, as well as other major moments in Spider-Man lore that modern comics radically recontextualized.
Alicia Masters Was A Skrull
A major Marvel Comics retcon that potentially factors in live-action involves Alicia Masters. The longtime Fantastic Four ally and love interest of The Thing dated Johnny Storm in the 1980s. During this period, the comics revealed that Alicia was in fact Lyja, a Skrull operative in hiding and had been in disguise for years.
This undid many things from the best Fantastic Four 1980s comic books, including Johnny's marriage to Alicia. Fans later discover the Skrulls captured the real Alicia, but for some, it undermined a relationship many enjoyed. Lyja likely appears in the MCU eventually with the advent of the Skrulls, and possibly even in Secret Invasion.
Moira MacTaggert Is A Supervillain
2019's House of X #2 revealed that Moira MacTaggert had been a mutant all along and had lived numerous lives already. While that still ranks among the biggest twists in Marvel Comics history, her recent heel-turn into a supervillain determined to destroy mutants throws a longtime X-Men ally and supporting player into shadow.
Moira's ultimate goal shifts from saving mutants to destroying them after Mystique and Destiny strip her of her mutant power. This completely upends the radical and exciting new dynamic that House of X introduced.
The Crossing
Iron Man often acted arrogantly throughout his comic book career, but he still counts among the most heroic Avengers. That all changed in The Crossing, a 1990s storyline that retconned Tony Stark's character completely. This storyline revealed Stark had been an agent of Kang The Conqueror for years and made him a murderer.
Comic book fans know Kang The Conqueror manipulates history but this change rankled fans confronted with the idea that thirty years of Iron Man continuity had all been a lie. The comics swept this retcon under the rug just a few years later.
The Clone Saga
The Clone Saga pulled the rug out from under Peter Parker's comic book history. This 1990s storyline suggested that Peter had been the clone introduced in an otherwise small story in the 1970s. The 'real' Peter returned to take his proper place as Spider-Man. Things spiraled out of control, leading to over two years of confusing plot twists and turns.
In the end, numerous Spider-Man clones appeared, and Ben Reilly, the actual clone, became the Scarlet Spider. Though Reilly enjoys many fans, the storyline remains emblematic of a tendency in Spider-Man comics to undo the past.
Thanos Rising
Thanos' character and powers evolved over time - he once flew around in the Thanos Copter - so it's no surprise to Marvel Comics fans that his story changes. Some fans felt the Thanos Rising storyline pushed things too far by altering aspects of his origin, like his relationship with his mother, Sui-San.
This storyline changed her character so she reacted poorly to his unusual appearance and tried to kill him as a baby. This deviated from the original storyline, which Thanos co-creator Jim Starlin later retconned back into place in a subsequent storyline.
Magneto Isn't The Scarlet Witch's Father
Retcons often retcon each other. This happened with the Scarlet Witch and Quicksilver's parents. An early retcon established their father as the Golden Age superhero The Whizzer. This changed to Magneto, among the best X-Men villains. Recent comics changed it again, this time revealing The High Evolutionary genetically manipulated the twins.
This last retcon frustrated fans used to the idea Magneto fathered the powerful twins, especially as it disconnected them from their deep ties into mutant lore, including Wanda's violent actions in House of M.
Jean Grey Didn't Die
Jean Grey's death in The Dark Phoenix Saga remains among the most shocking deaths in Marvel Comics history. Her heroic sacrifice remains an indelible moment for many fans despite the fact that Fantastic Four #286 retconned it out of existence. This issue established that the Phoenix Force only impersonated Jean Grey.
The Phoenix Force placed the real Jean in suspended animation at the bottom of Jamaica Bay, where her space shuttle crashed in Uncanny X-Men #101, and she apparently transformed into the Phoenix. This instantly generated massive consequences, including disrupting Cyclops' marriage to Madelyne Pryor. The comics then turned Madelyne evil to essentially get rid of her.
Hail Hydra
Few characters in comic books inspire as much as Captain America. When Marvel Comics retconned that the iconic superhero had been a Hydra agent all along, it didn't go over well, leading Marvel itself to admit it was a bad idea. Though the comics later returned a good Captain America, the damage in some ways was done.
Changing an America icon into a tool for the people he fought from the beginning shocked many readers, and it seems Marvel learned no lessons from the Iron Man storyline in The Crossing.
One More Day
Spider-Man: No Way Home borrows a little from One More Day, which surprises some fans as this early 2000s comic book storyline remains controversial. In the comics, Spider-Man and Mary Jane make a deal with Mephisto to erase their marriage from existence in exchange for Aunt May's life, undoing decades of stories.
The change occurred thanks to an editorial desire to see Spider-Man unmarried and younger, but it upset readers who hold the Peter and MJ relationship close to their hearts.
Gwen Stacy And Norman Osborn
An even more egregious Spider-Man retcon involves Gwen Stacy and Norman Osborn. Osborn killed Gwen in The Amazing Spider-Man #121, a seminal issue and moment in Marvel history. A 1990s comic book storyline called Original Sin retconned the past so that Norman and Gwen had an affair before she died. They even had twins.
This mystifying revelation persisted for years until just recently when the Nick Spencer run hand waved away this affair and their children, though, like Ben Reilly and the clones, some elements remain part of the canon.