Most influential retcons

[twiztor]

Or Namor and Captain America never knew each other in golden age and Invaders is a retcon by Roy Thomas.

this is a fairly common mistake. While the Invaders series retconned a lot of things and told stories set in the 1940s, it wasn't the first time the heroes actually met up. All-Winners Comics #01 (Summer 1941) features a text story where all the main players at the time meet. This includes Cap, Namor, and the Human Torch, among others. in All-Winners Comics #19 (Fall 1946) we actually get a story featuring the team (although the bulk of it is told through individual stories, the team as a whole does bookend the plot. All-Winners Comics #21 tells a tale with a similar setup.

[gordonm]

Xorn changing from being only a disguise for Magneto to an actual seperate person.

[teapartyofthedead]

Magneto as a Holocaust survivor, the father of the Maximoff twins, and having a telepathic resistant helmet. Cap going into ice and Bucky being brainwashed into being Winter Soldier.

[Marvell2100]

Alicia Masters actually being a Skrull when Johnny married her.

Sentry.

[K7P5V]

The neural patterns of Ultron were based upon those of his creator, Hank Pym.

[Huntsman Spider]

The neural patterns of Ultron were based upon those of his creator, Hank Pym.

That retcon actually worked, I thought. Made a lot more sense than him just randomly whipping up artificial intelligence.

[mung]

The Terrigen mist being the source of Inhuman powers. Initially they had full control over who got what powers before it turned into a dice roll system.

[K7P5V]

That retcon actually worked, I thought. Made a lot more sense than him just randomly whipping up artificial intelligence.

Agreed. And it was all Kurt Busiek's idea.

[ChrisIII]

All nice picks. Also Hulk having multiple personalities and a healing factor is all a retcon. Paul Jenkins retconned it further as Bruce having thousands of personalities.

Bill Mantlo sort of brought in the whole thing about his past abuse being a factor in the shaping of the Hulk; I think it was mainly Bryne who hinted that the Grey Hulk and the Savage Hulk were not technically the same being at different stages-and of course Peter David went to town with that, although mainly he stuck with just the three core personas. Although Jenkins revealing that there were even more personas including a "Devil" one was kind of seen as being an iffy retcon at the time, it's certainly helped shape the current run pretty well.

[ChrisIII]

Little side note to that-should be noted that in some of the early Grey Hulk stories, PAD wrote the Grey Hulk as if he thought he used to be the Savage Hulk, just smarter and greyer (Doc Green apparentally is a smart Savage Hulk, though)