Most DC characters (Superman, Batman, Wonder Woman, Green Lantern, The Flash, Hawkman, etc) were created during in the 1930s and 1940s. At that time, a mandressed in the American flagtaking down Hitler was everything that readers needed. World War II ended, the interest in such things died down, and most comic books began to close or to move to other genres. And on a day unlike any other, Gardner Fox and Carmine Infantino created a new Flash, unrelated to the old one in everything except the name (and indeed, confirming in his first appearance that the old one only existed as an in-universe comic book character), and the super hero genre was reborn, followed by similar relaunches of old DC glories. Did you follow up here? Well, one day Fox wanted a cameo appearance of the old Flash, and wrote "Flash of Two Worlds". Flash (Barry Allen) appears by accident in another world, "Earth 2", where the original Flash lives. They meet, save the day, Barry comes back home, and that's it. That's it? Hell, no. The Pandora's box had just been opened. What happens with Superman and Batman, whose titles had never been cancelled? Which stories are in Earth 1 and which ones in Earth 2? If Barry knows Jay's secret identity because there are in-universe comics about the Earth 2 characters, what happens with Clark Kent and Bruce Wayne? And why stop it with 2 Earths and not create new ones? All this led to DC deeming that the multiverse had become too "convoluted" and torching the whole thing in Crisis on Infinite Earths, which failed spectacularly to make it less "confusing". Ninety percent of DC's continuity snarls and messed backstories can be traced back to the Crisis and its aftermath.
Green Lantern: The New 52 stated that the events of the War of the Light still occurred, but that would also mean that all the events leading up to it are also in continuity including Hal Jordan becoming The Spectre after Jim Corrigan gave it up, and then passing it onto Crispus Allen who helped the character during the various crisises (including subduing the red Butcher entity). In the New 52, the Spectre is once again Jim Corrigan, and has never stated to be any one else. The Alan Scott Green Lantern, who acted in a mentoring role to Kyle Rayner and is now located on a different Earth, which means that he now never helped during the Blackest Night or the Power of Ion storyline. Plastic Man first appears in Forever Evil, which also has Sinestro in his post Parallax-inspired Wrath of the First Lantern costume, which means it takes place after those events, but Plastic Man was present with Kyle Rayner when he was part of the Justice League while Hal Jordan was gone after Emerald Twilight. It just goes on and on.
Connected to the Green Lantern stuff above is Cyborg Superman, Hank Henshaw. He is essential in Hal's fall from grace, which was still canon in New 52 GL works. But in stories taking place in the present day, Zor-El (Supergirl's father) is Cyborg Superman and completely new to the world. Crap. Okay, let's just assume he's the second Cyborg Superman and Kara just doesn't know about the original because she's new to Earth... And then Superman: Lois and Clark happened and features the pre-Flashpoint Superman actively trying to prevent the New 52 Hank Henshaw from becoming Cyborg Superman in the present day when he returns from a space mission, so it's impossible that any Cyborg Superman existed when Hal turned evil! With the advent of Rebirth and especially Superman Reborn, Lois & Clark has been reworked and Henshaw is established as having been Cyborg Superman in the past and somehow was transformed back into his human form. He returns to his Cyborg Superman form using a gem... one he acquired in Lois & Clark when he was an ordinary human!
There have been at least three separate reboots of the series, that there have also been a number of smaller Cosmic Retcons that involved things like long-standing characters being retroactively replaced with entirely different people, and that DC Comics at one point featured two vastly different versions of the group simultaneously.
DC later attempted to fix this with Legion of Three Worlds, which teamed up Reboot Legion, the Threeboot Legion, and a new Legion that was almost but not quite the original (the five year gap version was simply ignored). The result was that the Reboot Legion and the Threeboot Legion were put on a bus, leaving only one Legion, the first one, with the addition of Gates, Bart Allen and XS. While this left the characters still reacting to stories that had happened in the 70's and 80's, the resulting history was considered relatively uncomplicated, at least by Legion standards.
Then New 52 hit, and after limping along for a while with poor sales, the final issue revealed the current Legion was actually the Legion of a world similar to Earth 2, although there are few differences and Word of God is that it's not actually Earth 2.
Post-Crisis Power Girl went through sooo much of this - she's a Kryptonian, she's an Atlantean - she's a weird metahuman - nobody knows. Finally, it was declared that she was a survivor of the pre-Crisis multiverse and her Continuity Snarl was the universe trying to "fit her in" and failing. Now she is considered to be the Supergirl analog of the original (pre-Crisis) Earth-Two. Yes, this means now there's a second Power Girl in current continuity Earth-Two. Writers just don't know what wasp nests to leave alone. And as of New 52, Power Girl is again the Supergirl from Earth-2, only it's a completely different Earth-2 from the previous versions.
In a related manner, Supergirl's continuity is also pretty tricky due to DC's decision to kill the original Supergirl (Superman's cousin Kara Zor-El) back in 1986, and then trying to replace her with different versions before returning to the original character concept. There have been several different characters to use the name, including Superman's cousin, a shapeshifting alien created by a good version of Lex Luthor from a pocket dimension, said shapeshifting alien merged with a human who is simultaneously an angel, a completely different version of Superman's cousin and Superman and Lois Lane's daughter from the future. Oh, and some of those might be the same people, and some of them, even post-Crisis, might never have existed. Then there was the time the alien shapeshifter Matrix was removed from continuity by Infinite Crisis without retconning Linda Danvers. However, if you consider only the current one, who is just Superman's cousin, it's not really complicated.
The issues with Supergirl and Power Girl mostly stem from the same source - post-Crisis on Infinite Earths, there was a huge editorial push to keep Superman truly the last Kryptonian (evil ones notwithstanding), so anyone who wanted to revive Supergirl or Power Girl had to jump through a bunch of hoops to establish they're not "really" Kryptonians. For better or worse, this was relaxed by the mid-2000s.
Superboy: John Byrne's reboot of Superman caused all kinds of problems since, for one, his Superman was never Superboy. No Superboy to hang out in the 30th Century with the Legion of Super Heroes. Then you have all of the splintering of Superboy - clone, Superboy Prime, Pocket Universe Superboy, etc.
While not as extreme as some of the examples here, Green Arrow has had numerous minor, but confusing, problems since Oliver Queen came back from the dead.
The problems began when novelist Brad Meltzer wrote a Green Arrow story called The Archer's Quest centering upon Oliver Queen going on a road trip with former sidekick Roy Harper to retrieve items that could be used to discover his secret identity. The problem with that is that Oliver Queen hadn't had a secret identity in years! In fact, in the Quiver storyline written by Kevin Smith (which came out less than a year before Meltzer's story) the main piece of evidence Batman used to convince a resurrected and amnesiac Oliver Queen that he HAD been dead was newspaper articles which used his real name while discussing his death.
Another problem was the revelation that the whole Archer's Quest was a ruse and that Ollie had really been trying to recover a photograph which proved that he had been present on the day his illegitimate son Connor Hawke was born and that Ollie, ipso facto, was a dead beat dad. The problem is that this scenario is completely implausible given the circumstances under which Ollie originally found out that Connor (who he had been traveling with for a while before his death) was his son - he had been told by the truth by his best buddy Hal Jordan, who was (at the time) nigh-omnipotent with the power of all The Guardians Of The Universe Minus One. For Meltzer's scenario to make sense, we have to believe that Hal Jordan is capable of being able to see the DNA of a person by looking at them but is unable to tell when his best friend is lying about having no idea he had an illegitimate son.
It got worse several years down the line when Judd WinickJossed a fan theory that sprang up to explain away the discrepancy. The idea was that Ollie knew about Connor and tried to do the honorable thing by proposing to Connor's mother but that she had (having always been portrayed as an independent, free-spirited hippie) rejected him because she didn't want to marry only because he felt guilty/didn't want to get tied down. Instead, Winick wrote a flashback scene where Connor's mom approached Ollie and was sarcastically wished good luck in trying to prove the baby was his in court. This scene apparently took place BEFORE the shipwrecking incident which inspired Ollie to become Green Arrow, as he tracks her down once he gets back to civilization and is there to have his photo taken with Connor before he has a fight with Sandra and walks out of her life again.
What makes this truly awful is this scene was meant to bookend the excellent Green Arrow: Year One mini-series by Andy Diggle. Suffice it to say that Green Arrow fans who have read that book find it hard to believe that the man Oliver is at the end of the story would ever abandon a child in need, much less his own son.
Speaking of romantic problems, Judd Winick did a major disservice to the character when he decided to join Green Arrow and long-time girlfriend Black Canary together again off-camera, only to break them up. He did this by having Green Arrow suddenly decide to have a one-night-stand with the niece of a friend, despite the fact that Ollie was ready to propose to Black Canary not a few months earlier in the final chapter of The Archer's Quest by Brad Meltzer! Indeed, the dialogue in the scene where Ollie nearly proposes suggested that he and Dinah had gone out a few times since his resurrection but that she wasn't ready to date exclusively, let alone get married.
Judd Winick also caused problems with his Heading Into The Light storyline, which was meant to be a lead-in to Infinite Crisis. In the end, there were so many issues with the storyline that DC Comics had to retroactively declare that Heading Into The Light took place AFTER Infinite Crisis, even though the story ends with a wounded Oliver Queen having visions of himself in other realities.
Some problems also sprang up over the issue in which Doctor Light drained Kimiyo Hoshi of her powers. She appeared in Infinite Crisis and Birds of Prey with her powers intact, while other stories ran with the premise of having her powerless. This was eventually handwaved in an issue of Justice League, which had Kimiyo mention that while she still retained her abilities, they were now malfunctioning and only worked on certain random occasions.
The Bat-Family has been rendered a near-complete mess thanks to the "soft reboot" of the New 52, where the Broad Strokes of previous continuity have been kept in numerous cases until explicitly contradicted. Among the changes are a compressed timeline, where superheroes have only been widely known to the public for five years. The problem is, Batman and company get up to a lot, and the franchise's reliance on sidekicks makes the passage of time a bigger factor than with other heroes, so cramming all of Batman's history into five years seems outrageous at the best of times. A Hand Wave is attempted by having Batman operate in secret an extra year before the rest of the hero community, but this only helps so much:
Batman's eleven year-old son Damian has been retained. Damian was originally conceived after Bruce had become Batman, and it was reasonable for Bruce to have been Batman for at least eleven years in the previous continuity, but not in the New 52. Bruce is said to have operated for about five years at the start of the New 52, and by then Damian was already well into his tenure as Robin. The situation was left vague (Batman and Robin showed only four of his birthdays without specifying his actual age) until it was eventually implied that Damian was conceived after Batman's birth and that his mother Talia aged him artificially using technology from Apokolips. However, DC Rebirth later showed Damian celebrating his thirteenth birthday, which messed things up a bit. It would all prove moot after Superman Reborn expanded the timeline of the New 52 —and thus how many years Batman was active — which makes his age plausible.
Metamorpho mentions in an issue of Batman, Inc. that he used to be a member of the JLA. This was a Continuity Nod to Morrison's JLA run, which no longer exists in the current canon. It's also stated in a later issue of Justice League that the team's membership did not change at all during the 5 year Time Skip aside from a brief inclusion of Martian Manhunter, meaning that there's no way Metamorpho was ever part of the group.
In the previous continuity, Batman is "killed" by Darkseid using a special ability called the Omega Sanction, which actually unsticks Batman in time. During the year where Bruce slowly struggles back to the present, Dick Grayson becomes Batman and has Tim Drake move on to become Red Robin while Damian becomes the new Robin. The New 52 retains Bruce's disappearance and Dick and Damian's tenure as Batman and Robin while saying that Tim never took up the Robin mantle and immediately became Red Robin out of respect for Jason Todd's death, but leaves out the details on that time period until Batman and Robin eventually managed to simultaneously claim that: 1. Darkseid himself has not attacked Earth since the first arc of Justice League and 2. Batman was still killed by the Omega Sanction.
At the start of the New 52, it was established that Tim Drake had been Robin and was now called Red Robin. Then Teen Titans eventually contradicted this by saying that he was never Robin, and that he started as Red Robin. Then Rebirth happens, which is already weird with continuity, but had Bruce specifically state that Tim was never Robin... then Tim recounts his origin in a later arc, it's his pre-New 52 origin, complete with him having been Robin!
At the start of the New 52, Michael Lane is the current person going by Azrael, after Jean-Paul Valley, who was the second Batman. However, years down the line, Valley was introduced, and had only recently taken up the Azrael moniker...
And Valley is established as having had no interaction with the Bat-Family up until that point, meaning he could never have been Batman! Sure, fine, whatever... except a later arc, "I am Bane" has Bane invoke the story where he broke Batman's back and Valley became the new Batman, "Knightfall"!
Dark Victory caused one as Arnold Flass from Batman: Year One is among the Hangman's victims. However, while that story is set in the past (with Gordon still being married to his first wife, Barbara), the earlier-published Batman: Legends of the Dark Knight Annual #2 shows Flass alive and well when Jim Gordon and Sarah Essen get married.
Catwoman: When in Rome, which takes place during DV, caused another. While Infinite Crisis would render it moot by restoring Wonder Woman's status as a founding member of the Justice League, at the time, the status quo was that she's only become active around the time of Legends. The problem? Even with the idea of Hippolyta going back in time to fight in World War II during her tenure, Cheetah appears in the story — and is both clearly the Barbara Minerva werecat incarnation and explicitly stated to have fought Wonder Woman before in the story.
In the first issue of the New 52Teen Titans, it was stated that Tim Drake kept his history as Robin and that previous iterations of the team existed, with references also being made to past Titans teams in Red Hood and the Outlaws. Come the zero issue of Teen Titans a year later, and Tim's been retconned to have always been Red Robin and this is the first team of Teen Titans, with the collected edition of the first Titans arc outright removing the details that were retconned out. And as for the previously mentioned members of the Titans in Red Hood, so far the word is, more or less, that Dick Grayson, Starfire, and Arsenal (and possibly some others) hung around with each other, but never called themselves any team name.
The references to Gar (Beast Boy) and Garth (Tempest) made by Arsenal in Red Hood can now be considered retconned out, as Garth was later introduced as an infant in the Aquaman title and Gar is a hero just starting out in New 52 continuity (and also red-skinned and red-haired, as opposed to his classic green look). An early appearance of Gar (with green skin and hair) was also edited out in the TPB run of Teen Titans, along with a cameo of Miss Martian (who was recolored to be a white blonde girl).
A Teen Titans arc also featured the 70's Titan Lilith rebooted as a villain, with some of the team members seeming to know who she was. This would have been even more of a continuity snarl had Scott Lobdell been allowed to go through with his original plan: The villain was going to be Raven, who was responsible for founding the '80s team of Titans- who now no longer existed.
Speaking of Garth, he was actually later reintroduced in the pages of Aquaman as an adult, with no explanation given for the "infant" comment. However, he isn't a superhero, and is an Atlantean soldier who goes by the name Tempest.
The removal of the Titans' history also led to a comment in Batwoman being altered, where Bette Kane mentions that she'd been a member of the team and had fought Deathstrokenote although this itself had to be a retcon, as Flamebird never got such a chance or was on the main team. Of course, she could have also been bragging. The Titans reference was completely erased in the TPB edition.
All of these snarls from the New 52 reboot lead to the creation of Titans Hunt (2015), which is designed to not only give readers the "lost" Titans team mentioned, but also rerail personality traits lost with Donna Troy, Garth and Arsenal.
In the Day of Vengeance mini-series produced around Infinite Crisis, Bill Willingham had The Spectre murder the Lords of Order and Chaos, T'charr and Terataya. This caused Hawk and Dove to instantly become depowered. All other writers proceeded to ignore this for later stories, although it created confusion among fans as to how Hawk and Dove could still have their powers. T'charr and Terataya's deaths were a bit of a snarl in the first place, as both had previously died in the '90s Hawk and Dove series— and had their powers sealed into the two heroes. Holly Granger's existence is another snarl in H&D history, as Dawn Granger was always said to be an only child in the '90s series. Tellingly, after Holly died and the New 52 reboot happened, it would appear that Dawn is back to having no siblings.
Countdown to Final Crisis is absolutely full of this. It laid claim to being "the spine of the DC Universe," meaning that it referenced many other events going on at the time. This meant that if two events were running simultaneously, Countdown would claim they occurred simultaneously... even if that was impossible. One fan summed it up pretty well.
if you read The Lightning Saga in JLA #8-10/JSA #5-6 <6-8.07>, it appears to take place as an unbroken sequence of events. Likewise, if you read Amazons Attack (and if so you deserve great sympathy), it appears to take place as an unbroken sequence as well, dominating everyones attention for several days. But if you read Countdown, and assume (not unreasonably) that it relates events in at least roughly sequential order, then based on when it chooses to have scenes tying in to these other storylines one would have to conclude that the JLA and JSA teamed up and set out to find the Legionnaires... then put that on hold when Diana got kidnapped and the Amazons attacked DC... then (somehow) put that on hold to get together again for the climax of Lightning Saga and Wally West's return (which All-Flash #1 <9.07> tells us corresponds with Bart Allens death), and then all headed off to Keystone City for Barts funeral... before returning to DC and turning their attention to the Amazons again... and then left that alone again (with no sign of any resolution) and turned to other business like, e.g., going to work at the Daily Planet or spying on Mary Marvel... all before the final scene of Lightning Saga in which the Legionnaires leave our era. Oh, and all in the space of about four days. Make sense to you?
Speaking of the Flash, there's the West family in the New 52. Initially, Wally West was the thirteen year old nephew of Iris West, son of her brother Rudy West, as he had been pre-Flashpoint. He had a Cool Uncle in the form of Daniel West (the Reverse-Flash). However, DC Rebirth featured the return of the original Wally West, who is established to be the son of Rudy and Mary West as he'd always been, and both Wallys were named after their great grandpa, Wallace. Wally West II (the New 52 Wally) is retconned into being the son of Daniel... except Daniel very clearly established that he's, at oldest, about 24 when then-current stories take place, as he was caught committing a robbery by the Flash on his eighteenth birthday. Meaning he had a son at the age of eleven!
When Bart Allen became the fourth Flash, it's shown that Barry Allen, the second Flash and Bart's grandpa, had an active role in Bart's upbringing in Bart's series The Flash: Fastest Man Alive. This flies in direct contradiction with what has been previously said: Barry died before seeing his kids reach adulthood, never mind Bart. This might be explained with Infinite Crisis happening and retconning some of Bart's upbringing... but when Barry returns in The Flash: Rebirth, Bart is outright hostile towards him and the subsequent Flash series has him try to be closer to Barry because they were never close. Given the unpopularity of The Flash: Fastest Man Alive, it's likely that Geoff Johns (the writer of The Flash: Rebirth and the subsequent Flash series) just ignored its retcon, but it's still jarring if you read them back-to-back.
All of which seem to stem from Morrison's apparent uncertainty of whether or not his run has connections to the wider DC Universe. In Blackest Night, Bruce Wayne's skull is dug up and transformed into a Black Lantern to invoke an emotional response in all of Batman's allies while Dick Grayson is busy attempting to ward off all the undead villains that are attacking Gotham. Meanwhile, in Morrison's run, the Blackest Night is apparently not happening in Gotham or in London as Grayson had transported Wayne's body (which is kept under Wayne Tower as opposed to the unmarked grave all other DC works claimed it was buried in) to the latter in an attempt to resurrect Batman using a Lazarus Pit.
Batman #700, "Time and the Batman" features Chief O'Hara alongside Commissioner Gordon when they come to the aid of Batman, Robin, and Professor Nichols. The problem? Post-Crisis, the death of Chief O'Hara kicked off the events of Dark Victory, his death being a full year before Dick became Robin.
The whole deal with the X-Men comics. A lot of it is the Kudzu Plot started with Claremont, but a lot of it comes from the loads of Ret Cons and counter Ret Cons.
One example is Jean Grey. Until the late 1990s, it was (relatively) simple. Jean was Jean. Phoenix was (retconned into) a cosmic entity that took her identity, and Madelyne Pryor was her clone. Then those Running the Asylum couldn't get that straight, and turned it into this. The Summers' Tangled Family Tree got worse and worse from the 1990s onward.
During Grant Morrison's run on the book, the X-Men travelled to China where a mutant named Xorn was held prisoner, released Xorn, and took him in as member of the team. Xorn turned out to be Magneto in disguise. The degree to which this made sense is debatable (since when does Magneto speak perfect Chinese? Why didn't Wolverine smell him?) but it was at least easy to follow so far. But then Magneto started doing drugs and herding people into ovens, and when Morrison left the book, the remaining X-Book writers couldn't retcon him as an impostor fast enough. So it was someone pretending to be Magneto pretending to be Xorn. Then it turned out there was another Xorn, who was the brother of the fake Xorn. There's a reason they don't mention Xorn much these days.
How bad has it gotten? When Xorn reappeared on a team of Future X-Men traveling back in time for the Battle of the Atom, readers immediately assumed there was a surprise identity to be revealed. And they were absolutely right; this version was mind-controlled grown-up Teen Jean from the O5 team.
Somewhat amusingly, this trope is actually the reason Claremont was kicked off his second run on the title, as EIC Joe Quesada felt that his utter devotion to every minute detail of continuity made his stories nigh-incomprehensible.
An example of a Continuity Snarl only half caused by Claremont is that of Nova Roma and its inhabitants. As originally conceived by Claremont in the pages of New Mutants, Nova Roma was a Lost World city hidden deep in the Amazon jungle and founded by the immortal Vain Sorceress Selene during the days of Ancient Rome. One of its inhabitants, Magma, was established as being of blood relation to Selene (called her granddaughter, though more likely her descendant). But then years after Claremont left the title, new writer Fabian Nicieza came along with Retcons that Nova Roma was not actually ancient at all but merely a sham city full of people Selene had brainwashed, and Magma was in fact a British mutant named Allison Crestmere with no blood relationship to Selene. The two camps went back and forth on this after Claremont came back, until the continuity was so utterly wrecked that modern stories featuring Selene or the New Mutants tend to ignore both Nova Roma and poor Magma entirely.
During Marvel's "Acts of Vengeance" event, Wolverine fought Tiger Shark during his book's current story arc at the time. A few issues after that it showed while the other X-Men were doing at the time, and showed it was during the team's days in Australia. However, in their own book the X-Men team in Austrailia sans Wolverine had disbanded, had their memories wiped, and been scattered across the globe living under new identities long before the "Acts of Vengeance" crossover. It wouldn't be so bad if it wasn't for the fact that the X-Men's tie-in issues for the crossover were a key development for Psylocke, as it was during this event where she had had her mind transferred to the body of a Japanese assassin, a body she's stayed in since. So in X-Men after the crossover she had her new body, while in Wolverine's book she still had her original body after the crossover. Whoops.
The Mighty Thor has been prone to this, mostly because different writers draw on different myths. The current line of thought seems to be "he's a god and contradictory stories about him can all be true, somehow". Examples:
An early "Tales from Asgard" story shows Thor commissioning the creation of Mjolnir personally. Later stories have Odin using Mjolnir well before Thor was ever born.
Odin's wife was named Fricka or Frigga in early stories, until fairly recently when they decided she was Freyja. Yes, Frigga and Freyja are two distinct mythological figures, but scholars argue they may not always have been, so this wouldn't be too bad... except Freyja had already been established as separate from Frigga in the comics hundreds of issues prior note Plus, the Enchantress had previously been hinted to be Marvel's version of Freyja. Oy. . Not helping matters is that our new "Freyja" looked and acted rather differently from the Frigga we knew, yet the writers insist she's the same character.
Jason Aaron's run has had a severe case of this regarding Loki. Loki is presented as something of a Momma's Boy, working as Frigga's secret double agent to undermine Malaketh's reign, while petitioning the new Thor to give them a chance, noting the fact they want to make their mother proud. Except, over the course of Journey into Mystery and Loki: Agent of Asgard, its repeatedly shown that Frigga was terrible to Loki, blackmailing his innocent child self to work as Asgard's secret agent first, then betraying his teen self to trap him into his villainous destiny, which culminated in Loki, after being reincarnated as the God of Stories, makes it clear that he does not give a damn about what she thinks of him, only his friend Verity and his brother Thor. Come Aaron's run, and its never once mentioned what happened to Verity, Loki almost never mentions their new title as God of Stories, and personality-wise, Loki acts more or less like a mix between his old evil self (who's been dead and gone for years) and his MCU counterpart, rather than either how they were recently (a Brilliant, but LazyDeadpan Snarker with good intentions, haunted by guilt, and a love for modern Midgardian customs), or the God of Stories Loki (a Chaotic Neutral Cloud Cuckoo LanderMeta Guy trickster who spoke like a Brit punk). Ultimately, it comes off as if Aaron's not even read the other books, despite the latter having been going on during his run and having had crossovers with it.
Is Marvel' Superman equivalentThe Sentry a Silver Age hero who erased all knowledge of his existence so an evil being called The Void would not exist? Or is he a superhuman with mental problems who read a comic book and adopted the identity? Is he the results of Super Serum experiments with The Void being actually a part of his fragmented mind? Who knows?
When Paul Jenkins wrote the original Sentry miniseries, he was very aware of all the problems involved in retconning a Superman-level Flying Brick with godly reality-warping powers into the Marvel Universe. The series actually explored this theme at some length - and ended in a way that should have pushed the Sentry offstage forever. But the Bullpen just couldn't resist bringing him back about three times too many.
One of the original reasons for creating the Ultimate Marvel universe was to avert this trope by creating a blank slate free from the decades of continuity the main universe had built up. This didn't stop Ultimate Marvel from generating Continuity Snarls of its own.
One snarl concerns its version of Iron Man. The Ultimate Iron Man miniseries by Orson Scott Card, while good on its own, depicted Tony Stark as superhuman born with a healing factor and his brain distributed throughout his entire body. Since the Ultimates series depicted Tony as the more traditional nonpowered genius in power armor, this caused issues. Another origin story featured in an issue of Ultimate Marvel Team-Up created further problems. Although Brian Michael Bendis, who wrote the latter story, has suggested that some of Ultimate Team-Up is dubiously canon, it remains to be seen how or if the former will be reconciled.
The Ultimate Iron Man miniseries was later retconned to be an anime about Tony Stark's life that ignores the truth in favour of bizarre sci-fi, though that didn't stop the 'distributed brain' thing from showing up elsewhere.
In Ultimate Spider-Man and The Ultimates the Fantastic Four are referenced and Reed Richards is a notable enough scientist to have a building at ESU named after him, but very early on in Ultimate Fantastic Four, before the team comes together, there are references to The Ultimates. This is because of a change in plans. Originally, the Fantastic Four we were seeing in Ultimate Spider-Man and The Ultimates were going to be adults, while Ultimate Fantastic Four would take place a decade or so in the rest of the line's past, establishing the FF as the first super heroes and cornerstones of heroic society in the Ultimate Universe. The plan got muddled and changed, but it's very apparent when Sue Storm, 16-ish here, shows up during Ultimate Spider-Man's "The Clone Saga" and is clearly in her late 20s/early 30s.
Ultimate FF: While revealing that the Doom who kicked off the events Ultimate Power, The Ultimates 3 and Ultimatum and died was really Mary Storm, a new one was created: namely Doom calling out for Storm in the last of those events after Magneto's actions resulted in Latveria being frozen and is additionally compounded by Mary attending her ex-husband Franklin's funeral, which takes place after the Thing killed the Doom who caused those events.
The Destroyer is a Golden Age Timely Comics character who got his powers from a Super Serum. That's about all anyone can agree on about him. For instance, is he named Keen Marlow or Brian Falsworth? If he is Keen Marlow, how do you spell it? Keen Marlow, Keene Marlow, or Keene Marlowe? Or is Keen/e a nickname, and his real name is Kevin Marlow? If he's Brian Falsworth, where did the name Keen Marlow come from? Was it an alias created by Timely Comics for their stories? Was it a pseudonym he used while working as a spy? Did he ever go by that name at all? Finally, did he begin his Nazi-punching career before or after Captain America, and if after, was the serum used to create him based on the one used to create Cap or not? *Whew!*
The Crossing, a Crisis Crossover featuring The Avengers, is such a snarl that even Linkara refuses to review it, partly because it's nearly impossible to tell when it begins or ends.note It has since been released in trade paperback form, a copy of which Linkara's holding front and center in his 2016 opening credits, so it's now under consideration for a future review. Basically, Iron Man turns evil and helps Kang try to take over the world, but who is on whose side changes from issue to issue. Eventually, Kurt Busiek rendered the whole thing moot in Avengers Forever by stating that the entire thing was Immortus trolling the superhero community so that they'd leave other planets alone, and that almost everyone involved was a Space Phantom. After the Heroes Reborn/Heroes Return debacle, the Crossing was pretty much never brought up again.
Busiek also used Avengers Forever to untangle the very snarled continuity of The Vision. For decades, the Vision's origin story had him being created from the remains of the original Human Torch. But after the writers of West Coast Avengers decided to re-introduce the Torch to modern continuity in the late 80s, the Vision's origin was retconned, invalidating a lot of stories and raising a lot of questions about where the Vision did come from. Enter Busiek, who explained everything by having Immortus use a time-altering MacGuffin to change history, so that two contradictory events, the Torch being rebuilt and the Torch not being rebuilt, both happened at the same time. Neat, huh?
The symbiotes. First, the Venom suit was just an alien costume. Then it was retconned into being alive. Then, when the writers wanted to turn it into a villain, it was retconned that the suit made Spider-Man go insane and he had to get rid of it (originally, he was trying to destroy just because it was attaching itself to him, which is a bit harsh for a guy like Spidey). It was later shown that the suits fed off strong hosts as a sort of Social Darwinist. Then it was revealed to feed off negative emotions such as hate and anger. Then they were shown to live in the Negative Zone... no wait, there was a separate planet of them. Oh, and Toxin proved that not all of them are born evil after all. Oh, and Carnage has had about three symbiotes get destroyed but no one ever remembers those stories.
Who is the Hobgoblin really? The character was created by writer Roger Stern who strung along the mystery of his identity, dropping clues here and there. According to him, when he created the Hobgoblin he didn't have a set idea on who he was and only shortly into it did he decide it was a character he had introduced in a smaller title called Roderick Kingsley. Then he left and told his plans to his successor Tom Defalco who didn't like the culprit and Stern told him that he had his consent to come up with someone else. Later writers and editors felt that the Hobgoblin mystery was itself compelling and so spun wheels and Red Herring to extend the story forward, until they and readers got bored and frustrated, and finally it was revealed that Hobgoblin was Peter Parker's friend Ned Leeds, who had already been Killed Off for Real when this reveal happened. It is no wonder years later Roger Stern was allowed to return to the subject in a miniseries which was essentially a Fix Fic in which Stern gave the identity to the person he'd intended all along, and established that Leeds had been brainwashed into acting as a stand-in who was later sacrificed so that the original could retire. It helped that Stern had, in fact, established Hobgoblin's use of impostors during his original run.
As a consequence of dropping in the middle of several large, running storylines, Marvel's Avengers NOW! relaunch has resulted in some major continuity issues. The biggest one involves the plot point about Steve Rogers experiencing rapid aging and passing the Captain America mantle to Sam Wilson. This particular plot thread happened to come about at the same time Jonathan Hickman was prepping his big "Time Runs Out" crossover between The Avengers and New Avengers, the main crux of which involves the war between Steve's team (the Avengers) and Iron Man's team (The Illuminati). Despite the fact that "Time Runs Out" explicitly shows a still-young Steve vowing to hunt down Iron Man and his teammates (meaning the conflict definitely began before Steve was aged), other stories like Axis show the elderly Steve still working side by side with Iron Man without any animosity between the two.
Brian Michael Bendis's work had tended to have some of these—but a pretty big example is Avengers Disassembled—in its very premise as some years before, The Vision and the Scarlet Witch remembered their sons without either of them losing their sanity and the premise is dependent on Wanda never remembering them before the story. There's also Doctor Strange saying that there was no such thing as "chaos magic"—despite having used it himself in his own solo series. That latter one was later explained by Chthon himself, who said that the Sorcerers Supreme and others had tried to limit his power by saying that there was no such thing as chaos magic.
The first arc of the Avengers Assemble comic also has some issues: Iron Man also says "We barely know anything about Thanos" — despite the Avengers fighting him on numerous occasions (including partaking in the fight in The Infinity Gauntlet and its sequels) and having extensive files on him, and Thanos debuting in an Iron Man comic and thus Tony Stark himself was the very first Avenger to do so. Another thing was stating Thanos' goal was "He wants the Earth. He's always wanted the Earth," when in nearly every single prior encounter Thanos has ignored the Earth entirely.
This can be a problem when a character who was seemingly killed off is later revealed to have actually survived:
After The Wasp's death in Secret Invasion, Hercules ran into her in the Greek afterlife, where she was shown alongside a number of other dead Marvel characters. However, it was later established that she'd never actually been dead, and that the explosion that seemingly killed her had actually just trapped her in the Microverse.
In the X-Statix Presents: Dead Girl mini-series, Mockingbird appeared as a supporting character in the afterlife. The above-mentioned Secret Invasion later established that the Mockingbird who had died all those years ago was actually just a Skrull impostor who had replaced the real Mockingbird after she'd been kidnapped.
During J. Michael Straczynski's Thor run, he had Thor meet the departed spirit of Captain America, who had recently died at the end of Civil War. The later Captain America Reborn mini-series established that the gun Cap had been shot with was actually a special device that merely sent him through time rather than killing him.
The Spider-Woman named Jessica Drew has at least three different origins:
Originally, she was one of the High Evolutionary's experiments in engineering new humanoid species from animal stock; in Jessica's case, she was a spider artificially evolved into a perfectly humanoid form.
Then she became the daughter of a pair of scientists who was poisoned by the uranium deposits near their lab; her father injected her with a serum made from spider genes in an effort to cure by infusing her with the resistant to radiation possessed by spiders. He then put her in a "genetic accelerator" to enhance the serum's progression, which caused her to A: gain spider-powers, and B: rapidly age until she was a child in the body of a young adult. The "spider evolved into a human" backstory was retconned as a false set of memories implanted in her.
Another comic, "Spider Woman: Origin", retcons it again, most notably stating that whilst her parents were experimenting on a way to graft useful genes from spiders to humans, Jessica's pregnant mother was zapped with a splicing beam and Jessica was imbued with spider-genes in her mother's womb.
The canonicity of TaleSpin and Chip 'n Dale Rescue Rangers in relation to the wider Disney comic universe is unclear at best. The Crisis CrossoverLegend of the Chaos God seemed to show Talespin as taking place about fifty years before the "present-day" of both DuckTales and Chip 'n Dale Rescue Rangers (which in itself is not without its problem, as Rescue Rangers was previously implied to be a reboot of Chip and Dale in a universe stripped of Donald Duck and Co. entirely). But then a Disneyland stage play showed Don Karnage from TaleSpin and Fat Cat from Rescue Rangers cooperating on a heist in the present day, with no time travel in sight.
Donna Duck was initially introduced in the cartoon Don Donald as Donald Duck's girlfriend. The cartoon's ending seemed to show them breaking up, but Donna herself, aside from her name and appearance, looked identical to the then-not-created-yet Daisy Duck. Indeed, Daisy herself was soon established as Donald's steady girlfriend (and let's not go into how/when the two met, please). But the British Mickey Mouse comics kept using "Donna" as the name of Donald's usual girlfriend for several years. It became somewhat accepted that "Donna" was just an older name for Daisy, like "Dippy Dawg" for Goofy, and the two had always been one and the same. But that truce was not to last, as Al Taliaferro's comic strip showed Donna and Daisy meeting (seemingly for the first time), with Donna referred to as Donald's former girlfriend. Alright, fine. This became the norm as well, and was referenced in a couple of European stories too. But then one other Italian story showed Don Donald to not be canon at all, and, invoking a peculiar brand of Literary Agent Hypothesis, showed that the cartoon was an in-universe comedy film in which Donald and Daisy starred as actors. To mess it all up even further, Don Rosa foolishly released Word of God (or Word of Dante depending on who you are) claiming that Donna was actually Daisy Duck's sister and the mother of recurring characters April, May and June. You may recall that Taliaferro's strips, the only preexisting soruce to establish that Donna and Daisy were separate entites, clearly showed they didn't know each other. (To add to the insanity, the Inducks database lists the British Donna Duck and Taliaferro's character as separate ones, for whatever reason.)
As if this was not confused enough, Don Donald was released on VHS several times, billed as Daisy's first appearance (once on a tape entitled Starring Donald and Daisy). Leonard Maltin notes Don Donald as Daisy's debut in his intro for the first Walt Disney Treasures Donald volume, but concludes that she does not "become the Daisy Duck we know and love" until later on.
Firstly, their (only we'll get to that) DuckTales story arc, "Rightful Owners", was an ambitious attempt at Canon Welding tying the classic comics and DuckTales (1987) into a single continuity; the comic finally showed Donald returning from the navy, thus serving as a finale for the show. Alas and alack, they messed some of it up. Most glaringly:
The 'Golden Fleece' scene unconvincingly tried to mix the comic's backstory and the (completely different) cartoon, where the Dragon and the Fleece itself are in a state that fits the ending of the comic, but the Harpies guarding the two are the ones from the cartoon, who, on top of everything else, act as though the Ducks only visited once, so you can't just pretend the cartoon and comic Harpies swapped jobs or something.
The Golden Helmet is seen in Scrooge's collection of artifacts, and Donald and the nephews acts surprised at this, as (they say) they last saw the thing when they threw it into the sea, obviously calling back to the original Barks story. (What Happened to the Mouse? is involved here, since obviously how Scrooge got the Helmet was going to be addressed had the comics had a long run.) However, this ignores the Don Rosa sequel to this comic where the Helmet was retrieved by Scrooge, in full knowledge of Donald, before turning out to be worthless in the end. This can't be a case of Canon Discontinuity on Rosa's stories, either, because Don Rosa's comics are explicitly referenced elsewhere.
Moreover, Boom losing their licence (presumably due to the aforementioned royally messing up the continuity) meant they had to wrap up their gigantic Canon Welding in four issues flat to lead into the Dangerous CurrencyCrisis Crossover with Darkwing Duck. As a result, the dozens of villains gathered by an unseen force turned out to have been simply rounded up by Rockerduck even though it makes no sense for some of them to be there since they don't have anything to do with the kind of job Rockerduck would need them to do
Then said Crisis Crossover rolled round and lo, it messed up the continuity even further. Mrs Crackshell now knew about Fenton Crackshell being Gizmoduck's Secret Identity, even though she knew no such thing; whereas Darkwing Duck acts surprised to discover it, even though he'd already found out in the original TV series. Because clearly this wasn't bad enough, they then confused up the lore surrounding the Tronsplitter, where, now, the original "soul" of the divided person is sent to a Pocket Dimension even as the split halves retain individual consciousness. You'd think Darkwing Duck would have mentioned something like that when being put back togehter the first time around, but nope. And to confuse things even further, this is, somehow, the same pocket dimension that Morgana McCawber was sent to by Duckthullhu for no discernible reason, really.
Wait, we're not through yet! You see, Dangerous Currency was released illegally by Boom, as they'd actually already lost the licence by the time they got around to printing the sorry mess of a story arc. So Disney, being pretty pissed off by this, proceeded to declare Canon Discontinuity onto Dangerous Currency. Specifically onto Dangerous Currency. Rightful Owners's validity was not addressed, and likely won't be now that future DuckTales comics will presumably be in continuity with the 2017 reboot, leaving the old show's continuity behind. Not to mention the hints about Dangerous Currency that had been dropped in still-canon previous issues of the Darkwing Duck comic. The 2016 comics looked like they were going to address the issue, with a (hopefully better) remake of Dangerous Currency but then that got cancelled, too, before it could contribute anything much.
The 2000 AD strip Strontium Dog. The guest appearance in Judge Dredd revealed that Johnny Alpha's adventures took place in the future of the Dreddverse, even though that was hard to square with Strontium Dog continuity (basically, where did all the Mega Cities go?) But then it turned out that didn't even matter, because the strip was rebooted in 1999, with the explanation that the previous run, Dredd crossover and all, was just a legend, and this was the true story. Which didn't stop spin-off strip Durham Red sticking with the original version. And then John Wagner returned to the original continuity, making the reboot non-canon...
Prior to the Continuity Reboot, the book had this problem early on, particularly with trying to adapt Sonic Adventure and Sonic Adventure 2. With Sonic Adventure, the big problems were that, beforehand, the comic was running with the idea of Humans Are the Real Monsters, depicting them as Four-Fingered Hands types that were destructive and war-like even with exceptions like Nate Morgan. Sonic Adventure had normal and accepting humans, which clashed with this. There was also the fact that Station Square was nice and clear, compared to the rest of Mobius. It was explained that this was a hidden colony of humans who survived After the End.
Then, there's everything involving echidnas and Chaos' origin. Echidnas first - when Knuckles first appeared, he was still Last of His Kind. When his mini-series was launched, it revealed his dad was alive. His series expanded that to saying there was an entire city of echidnas still alive, along with a group who preferred technology and more of Knuckles' family! Enter Sonic Adventure and the Knuckles Clan! How does a group of Mayan-inspired echidnas fit in with the technologically-superior ones we see now? Oh, they're a group of echidnas that decided to forego tech and turn native. For Chaos, the games proclaimed that he was trapped inside the Master Emerald and freed by Dr. Eggman. However, in the comics, the Master Emerald was never created until about a year or so before the game came out and Mammoth Mogul was stuck in it. So, they stuck Chaos inside a black Emerald-like item, then had Eggman shatter the Master Emerald, freeing Mogul.
The last 2005 BIONICLE comic, Fractures, ended with the six Toa Hordika marching into the Coliseum to confront the villains, with a note informing the readers that the Cliffhanger will only be resolved in the movie released later that year. The movie of course contradicted the comic's last few pages — only five of the Toa Hordika broke into the Coliseum, not to defeat the bad guys but to free their leader Vakama, who's become evil due to Roodaka's persuasion. In the comic, Roodaka never even meets him, although his turning evil is foreshadowed. The Hordika are also accompanied by the legendary beast Keetongu, who's only talked about in the comics, but doesn't actually appear. Some of the 2004 comics also had smaller inconsistencies with that year's movie — for example, did Vakama learn how to use his launcher as a jetpack and Matau his blades as wings before or after traveling to Ko-Metru?
Rogue Trooper was a strip about a genetically-engineered Super Soldier named Rogue who fights a one-man war on Nu Earth, accompanied by the digitised minds of three of his comrades in microchips attached to his helmet, gun, and backpack. Makes sense. At some point he met a woman of his race (for want of a better word) named Venus Bluegenes, who died. No problem. Later on, the series was given a brand new reboot, using Dave Gibbons' original vision, starring a new GI named Friday. So far so good. Then Venus got her own spinoff, but set in Friday's continuity. Uh, well, maybe she's an alternate version. Then Friday met Rogue and everything stopped making sense.
Kanan: Early in the comic, Caleb Dume is given a holocron by his master, Depa Billaba, shortly before Order 66. The indication is that this is the same holocron that Caleb/Kanan is shown to have in Rebels, 14 years later. However, in Kanan's first appearance in A New Dawn, set in 11 BBY, six years before the beginning of Rebels, it's stated that his lightsaber is the only thing he still owns that could reveal that he's a Jedi. The idea that he abandoned the holocron somewhere, then came back for it, is rather implausible to say the least.
Darth Vader: Dark Lord of the Sith: Palpatine questions Vader if he knows how Sith create their red lightsaber crystals, and Vader responds that he only knows that the method is called "bleeding the crystal", but it wasn't discussed by the Jedi. However, in Ahsoka, published before this comic started, Ahsoka, Anakin/Vader's former apprentice, not only knows what bleeding is well enough to explain it to Bail Organa, she knows how to purify corrupted kyber crystals. It's rather strange that Vader's ex-apprentice is privy to Jedi secrets that he isn't.
Vampirella has one almost as bad as Hawkman. She begins her origin as a Human Aliens vampire who comes from the planet Drakulon only to find the Earth is full of supernatural vampires. It's a cheesy fun juxtaposition of science fiction and fantasy you'd find in the 1970s. Then Harris comics retcons Vampirella's origins so she's actually the daughter of Lilith Brainwashed and Crazy into believing she's from the planet Drakulon. A remorseful Lilith raised her in the Garden of Eden to hunt monsters and her evil siblings. Lilith would then be retconned into a villain or a good guy who had villainous tendencies depending on the era. Oh and Drakulon would be revealed to be a place located inside Hell. It reaches its zenith in Dynamite Comics' 2011 book where Vampirella is revealed to have two mutually contradictory sets of memories about being a Human Aliens vampire and the daughter of Lilith. There's also ample evidence of both being true spread throughout the series with the opening stating she has a Shrouded in Myth origin. The 2016 comic has gone back to making her an alien with no mention of the other elements.