When DC wanted to reimagine the series, Hal Jordan (the Green Lantern) pulled a Face–Heel Turn and became a super-villain named Parallax, who killed all the other Green Lanterns. After this, Hal was replaced by Kyle Rayner, but eventually DC decided to bring Hal back. In order to smooth over his change with fans, it was revealed that he never was actually evil, he was possessed by a cosmic being of fear named Parallax.
The power rings' "yellow weakness." First, Green Lantern was weak against the color yellow because of a necessary impurity in his power, then it was revealed that the restriction wasn't necessary at all, it was something artificial the Guardians imposed on the Green Lanterns to keep them from going power-crazy. When Kyle became the only Green Lantern, the yellow impurity was removed. In Green Lantern: Rebirth it was retconned so that the yellow impurity was caused by the alien entity Parallax being trapped inside the Central Power Battery that gave all the Green Lanterns their power, and Kyle didn't have the yellow weakness because Parallax had been set free by Hal. Since then, the current manifestation of "the yellow impurity" is that the Green Lantern can only use his power against the color yellow if he knows the (most current retconned) source of the yellow impurity, and consciously overcomes his fear.
Following Green Lantern: Rebirth, the colored lanterns of the Emotional Spectrum are introduced to the mythos, having retroactively existed alongside the Green Lantern Corps. Their past can then be elaborated on with flashbacks or time travel, often sharing a history with the Green Lanterns.
Another rather controversial retcon by the same author as the Green Lantern example was in The Flash: Rebirth. Barry Allen came back (which was fine) but now instead of the previous "Happy Family" he had, his father was accused of the murder of Allen's mother. Really it was Professor Zoom, who went back in time and killed Barry's mom to ruin Barry's life.
Another Flash Retcon involved Wally West asking Spectre to erase the memories of his identity from everyone on earth after the new Zoom tried to kill his wife. Spectre did this but left a loophole so that certain characters would remember everything when Wally took his mask off or put it on in front of them. His wife left him for a while, but came back at the end of the arc. Hilarious in Hindsight when you realize it did the same basic thing Brand New Day set out to do (Make their identities secret and make them single, not that the last part was a reason for doing the Flash arc) and did it better.
The much-loathed Heroes in Crisis had a few, including that Roy's drug problems started with medication, that Doc Magnus loved Platinum, that Robin-expy the Protector was a recovering drug addict himself, that speedsters have to consciously exert control, and that Wally always remembered his kids when he returned.
Naturally, the main issue fans had with Heroes in Crisis, having Wally West accidentally kill his fellow heroes and cover it up were themselves retconned with The Flash (Rebirth) #761 stating the attempted cover up was the result of Reverse Flash screwing with that Wally's mind and completed by The Flash (Infinite Frontier) Annual 2021 outright stating Wally didn't even kill the heroes at all; the explosion was really caused by the Speed Force trying to kick a returning Savitar out of it. The end of "One Minute War" completes the trifecta by saying — barring Roy due to how close he was and knowing he'd come back, anyway, and Poison Ivy, due to her resurrection during the events of HiC — that no one died during the explosion, as Gold Beetle saved them and replaced them with clones.
The penultimate issue of The Flash (Infinite Frontier) reveals that Robbie Long, Cerdian, and Mr. Terrific's long-thought unborn son didn't die, but were actually saved by Granny Goodness to use against the heroes.
Deconstructed in Cary Bates and Greg Weisman's post-Crisis relaunch of Captain Atom for DC, in which the eponymous hero (Anti-Hero? Protagonist?) has one origin, which the military covers up, instead publicizing a "false" origin, which was Cap's pre-Crisis Charlton Comics origin. Later on, when Cap lost his powers temporarily, he wore the costumes that he had worn in the Silver Age, because, after all, the public in-story would be familiar with those costumes, having been told he used to wear them.
DC's 1991 event Armageddon 2001 turned out to be a huge mess at the end of the day and a major source of Character Development for one Hank Hall (Hawk of Hawk and Dove), which continued through Zero Hour until his death in the pages of the 2000s Justice Society of America relaunch. It also had the nasty effect of unceremoniously killing off the second Dove (Dawn Granger) in a cheap shock scene. However in the later pages of JSA, a big retcon by Geoff Johns would unfold: The woman who the JSA thought was a comatose Lyta Hall turned out to actually be Dove disguised by Mordru in some strange concealment spell (apparently they had to retcon Lyta to Dove at the time due to some issue with Vertigo too). The explanation of the retcon was quite convoluted and Squick: Monarch did not actually kill Dove, Mordru simply made an illusion to make Hawk think she was dead. Then Mordru possessed Hawk and made him rape the comatose yet still aware Dove, impregnating her with his child. So Dove was kept concealed and pregnant for who knows HOW long until she was found by the JSA, disguised as Lyta for some reason who was disguised as yet another woman, and yet she winds up strangely calm and relatively unaffected considering that she was raped and put into such a situation. And the baby? Wound up being a reincarnated Hector Hall. Not surprisingly, little reference has been made to exactly how Dawn cheated death ever since, she just did.
After her return, Dawn then mysteriously gained a younger sister named Holly, however this change was received even worse due to it contradicting various things in the '80s Hawk and Dove series, including Dawn being an only child and the powers being unable to pass on to anyone else. Unsurprisingly, Holly wound up becoming C-List Fodderdown the line as it seems no writer could figure out what exactly to do with her or how to portray her.
As noted, DC's All-Star Squadron are the Trope Namer. A book in the '80s, set during World War II, introducing a never-before-mentioned over-arching superhero group. It filled in a LOT of gaps in the continuity of the time, picking up dangling threads and plot holes (Why didn't Spectre & Superman just end the War? for example), and reviving many long-forgotten characters. Writer Roy (& Dann) Thomas had to work a little harder when Crisis on Infinite Earths hit, but still continued with Young All-Stars.
Just a year in and they're already contradicting themselves. Teen Titans had Tim Drake mention his time as Robin and that there had been prior versions of the Titans. When the trade paperback came out, this was revised with Tim always being Red Robin (never regular Robin, though still Batman's sidekick), and omitting mentions of prior Titans.
The Titans were also originally referenced in the Batwoman series, with Flamebird claiming to have been part of the team and having fought Deathstroke. This dialogue also found itself edited when it came time for the trade paperback to be released.
The thing with Tim never having been a Robin was left in when it came to the trades collecting Batman, with the Bat-Computer specifically mentioning it. Oops.
Convergence: Plastic Man and the Freedom Fighters retcons Plastic Man into being one of Earth-X's Freedom Fighters, when he'd never previously been a member. note Plas and the members of the Freedom Fighters are all Quality Comics characters, so including Plas on Earth-X makes it fully a Quality Comics world.
Black Canary naturally always had black hair, pre-Crisis and post-Crisis. She used to wear wigs but eventually just grew her hair out and dyed it. New 52 retconned her into being naturally blonde. Eventually that was retconned away, and Black Canary is back to bleaching her naturally black hair.
Starman retconned Happily Married Dinah Drake as having briefly cheated on her husband with (also Happily Married) Ted Knight. This was briefly alluded to in JLA: Year One, but otherwise, what with Dinah being dead, it rarely comes up.
The canary scream is either due to radiation, a wizard's curse, a metagene, or human experimentation involving aliens.
Captain Carrot has always operated under cartoon physics, but they were the burlesqued physics of Golden Age comics, not the outright Toon Physics of Looney Tunes as shown in The Multiversity. No previous incarnation of the character, even relatively recent ones, could possibly have survived decapitation.
During The Judas Contract arc of The New Teen Titans it was stated that Terra was doing everything by her own free will. In the 2000s, it was shown that Deathstroke had actually drugged her into behaving that way.
The Wally West introduced in the New 52, who has given a Race Lift, was revealed not to be the actual Wally, but one of Barry and Wally's relatives who was named the same.
The actual Wally? He'd been pulled into the Speed Force and is actually the original Wally.
The biggest change? The New 52 universe is actually the original universe, having been transformed by an entity implied to be Dr. Manhattan.
Superman's origin and past exploits all mostly happened, thanks to the pre-Crisis and New 52 Superman and Lois Lanes merging. The events of Superman: Secret Origin, his 80s and 90s adventures, his marriage to Lois Lane and other things all happened. The New 52 adventures happened while Lois was pregnant with Jon Kent and Jon was born in the Fortress of Solitude, not the Flashpoint Batcave as established in Convergence. The events of Superman: Lois and Clark are explained as part of a sabbatical where Superman focused more on being a father than a hero, allowing Lex Luthor to become his own Superman.
As a consequence, all heroes have aged up, having been active for 15 years, not 5.
Batman/Superman: World's Finest, much like it did with Supergirl, undoes changes made to the lore of Shazam! in the New 52, having Billy operate out of Fawcett again and having been a solo hero for a while before the forming of the Marvel family.
