The Marvel Cinematic Universe is an ever-expanding tapestry of movies and characters that are interconnected and plotted out years in advance. It is an unprecedented experience, one that is rewarding to its longtime fans and intricately detailed and satisfying to re-watch from its beginnings. But considering that the universe has been going on for almost 10 years, there's bound to be a few things that fall through the cracks. Whether it was lack of planning or a change in the grand scheme, some notions and segments in the Marvel movies had to be changed to fit the larger narrative.

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In the comic book world, a retcon is a change brought to something that was established previously, something that either changes it completely or simply adds a whole new wrinkle to it. You might have even recently heard about James Gunn considering retconning an element that was established in his first Guardians of the Galaxy movie for Vol. 3 -- something that has to do with one of his character's rap sheet -- but we don't yet know what that element is. But this isn't the first time a retcon has occurred in the MCU. Today, CBR lists 15 retcons that have happened over the course of the MCU's 10 years.

15 SPIDER-MAN APPEARED WAY BEFORE CIVIL WAR

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This big Spider-Man news came out just before Spider-Man: Homecoming hit theaters, when Tom Holland confirmed that the little boy we saw all the way back in Iron Man 2 during the final battle between Stark and the HAMMER Drones was in fact none other than Peter Parker. This little kid who, with an Iron Man mask and hand, stood up bravely against a drone that was coming for him.

This retcon actually makes a lot of sense when you think about it. Iron Man 2 was set roughly six or seven years before Captain America: Civil War. His age would add up, and it would add even more weight to how Peter looks up to Tony, as a scientist and self-made superhero... and all of that could be traced all the way back to the time Iron Man saved Peter, blasting the drone and telling him “Nice work, kid!”

14 THE MANDARIN ACTUALLY DOES EXIST

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The big twist in Iron Man 3 came when it was revealed that the man we thought was the Mandarin in his propaganda videos was actually just an actor named Trevor hired by Aldrich Killian. Killian proclaimed himself the real Mandarin, adding that the Mandarin was his creation. The twist was a great departure from the source material and it understandably angered a lot of fans.

But it would seem that Marvel Studios was listening. As an extra on the home release of Thor: The Dark World, there was a short movie titled All Hail The King starring Ben Kingsley as Trevor. This short movie actually went on to confirm that there was in fact a real villain named the Mandarin out there somewhere -- and he was very displeased that his name had been used in such a way.

13 EYE OF AGAMOTTO IN ODIN'S TREASURE ROOM

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Eagle-eyed viewers will have paid careful attention to every artifact that we saw in Odin's treasure room back in the first Thor film. While this one is more than easy to miss, it has still been confirmed that the Eye of Agamotto was in fact present in that sequence. As Marvel fans now know, the Eye is a very powerful magical artifact closely associated with Doctor Strange.

However, in the Doctor Strange movie, we saw that the Eye of Agamotto was in fact in Kamar-Taj, the home of the Ancient One and the place where Stephen Strange was taught the mystic arts. It's highly likely that the Eye of Agamotto was only included in Thor as an Easter egg for fans, considering that, at the time, Marvel Studios could never have foreseen that a Doctor Strange movie could actually get made, let alone succeed.

12 THERE ARE TWO INFINITY GAUNTLETS?

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The Eye of Agamotto wasn't the only artifact that was glimpsed inside Odin's treasure room in Thor. We also saw, for a brief second, an Infinity Gauntlet, fully assembled with all six Infinity Stones. In subsequent Marvel movies, we would come to know that the Infinity Stones are currently in the midst of being revealed and assembled by Thanos and his forces.

While the idea of two Infinity Gauntlets is not new in the Ultimate universe (eight infinity stones divided into two gauntlets of four), it seems that this is an idea that was partly brought into the MCU later on. However, it's doubtful that having two Gauntlets was the plan from the very beginning, considering that Thor was produced before The Avengers, and before Thanos was ever brought to the screen. Besides, why would Thanos break into Asgard to steal an Infinity Stone, when he could steal an entire Gauntlet instead?

11 STARK AND GENERAL ROSS TALK ABOUT A TEAM

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In the closing moments of The Incredible Hulk, audiences got their first taste of what a Marvel shared universe could offer. Audiences cheered when they saw Tony Stark first walk back on-screen after his successful first movie, telling General Ross they were putting a “team” together. While undoubtedly awesome, this scene turned out to go against the story that was ultimately set-up in the following movies.

For that reason, we got a short movie starring Phil Coulson and Agent Sitwell titled The Consultant that appeared as an extra on the home release of Thor that would give this scene a whole new wrinkle to make it fit into the timeline of the MCU. The short movie revealed that Stark was sent to actually discourage Ross from releasing Emil Blonsky -- The Abomination -- to the World Security Council to be a part of the Avengers Initiative.

10 ASGARD LIGHT SWORDS

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While not a big change that affected the MCU or that introduced something or someone way before we actually knew of it, this change to Asgard's armory still sticks out as a point of puzzlement in Thor: The Dark World. When we were first introduced to the Asgardians in Thor, we saw them wage war with their swords and spears, fighting like knights in medieval times.

Yet, when we saw them again in The Dark World, we saw that their weapons were different, that they had a light effect to them that sparked whenever they would spar. In the first movie's prologue with Odin, there were no such powers to their swords, but in the second movie's prologue, which featured Odin's father Bor, the Asgardians fought with those light swords. While not a big change, it still altered the way Asgardians were perceived visually.

9 TONY STARK “HAD” CAPTAIN AMERICA'S SHIELD

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Back in the very first Iron Man, the idea of having the Avengers team-up on-screen was just that -- simply an idea -- wishful thinking that depended on many factors. But that didn't stop Marvel from dropping a few Easter eggs into their movie. Chief among them was a very brief look at Captain America's shield in Tony Star's workshop.

However, the further expansion of the MCU meant that if there was a Captain America, he would be frozen in the ice with his own shield and so, in Iron Man 2, we saw Tony's shield once again, this time in plain focus, and it was made clear that this shield was only a little project Tony was working on for fun, and not actually the real deal.

8 THE RAINBOW BRIDGE IS... FIXED?

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The final act of the first Thor movie highlighted Thor's bravery and will to sacrifice what was most precious to him. When he was faced with the fact that the only way he could save Jotunheim from total destruction was by destroying the Bifrost, Thor didn't hesitate to do what was right. It didn't matter to him that Loki was telling him he would never see Jane Foster again. Selflessly, he acted as a hero.

The moment was strengthened by the notion that Thor was sacrificing his relationship with Jane, and how massive a deal it was to destroy the Rainbow Bridge. And yet, when Thor: The Dark World came around, that sacrifice didn't seem to mean much, because the Bifrost was there, fixed and working properly, with almost no mention of how it was rebuilt.

7 AGENT COULSON LIVES

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There haven't been many casualties in the Marvel Cinematic Universe yet, but the few there have been had a devastating effect on the audience. In The Avengers, Phil Coulson was the first major player and recurring character of the MCU movies to die. Considering that he was a likeable character from the start and that the audience came to like and depend on him throughout his appearances, this came as quite a blow.

His death was so important in fact that it fuelled the Avengers to put their egos aside and band together to stop Loki. But, seeing as he was such a popular character, Agent Coulson was brought back to life in the television series Agents of S.H.I.E.L.D. thanks to some alien technology. While we understand the need to bring a beloved character back, this retcon still lessens the loss established in The Avengers.

6 AGENT SITWELL AND SENATOR STERN ARE HYDRA

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Senator Stern appeared in Iron Man 2 as a representative of the government, spearheading the hunt for possession of Tony Stark's Iron Man technology. He was a nuisance, a politician made to be little more than a thorn in Stark's side, one who brought with him quite a few laughs. Similarly, S.H.I.E.L.D. agent Sitwell appeared both in Thor and The Avengers as a sort of right-hand man to Phil Coulson.

Both of those characters seemed to have little effect on the big developments of their own respective movies, but the plan changed when Captain America: The Winter Soldier came around and revealed that both of these characters knew each other as secret members of Hydra. While we can't imagine this was the plan for both of these characters from the start, this was a great retcon that worked because it somehow made sense.

5 THE MIND STONE WAS INSIDE LOKI'S STAFF

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Back in The Avengers, Loki was gifted a scepter by Thanos himself, a scepter that allowed him, at the slight tap of its blade, to take control of the minds of others. Powered by a blue gem, this scepter allowed Loki to recruit Dr. Selvig and Hawkeye, among many others. Many fans wondered if this blue gem could in fact be one of the six Infinity Stones -- the Mind Stone.

But this was never confirmed in The Avengers. After the idea of Infinity Stones was established in Thor: The Dark World, viewers would have to wait for Avengers: Age of Ultron for the titular villain to reveal that this blue gem was just a casing, hiding something much more powerful inside: a smaller, yellow colored stone hidden inside the blue gem – a yellow stone that would be used to bring sentient life to the Vision.

4 BLACK WIDOW AND HAWKEYE'S RELATIONSHIP

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Back in the first Avengers film, we got to see Natasha Romanoff and Clint Barton as a unit. We saw the bond they had, how devoted they were to one another, how Natasha was worried when she was told that Barton had been compromised. It was never plainly clear, but some viewers could definitely feel a romantic tension between the two.

More depth was added to that fire when viewers spotted the arrow necklace Natasha sported in The Winter Soldier movie. But all of those suspicions were thrown out the window in Avengers: Age of Ultron when it was revealed that Hawkeye had a secret family no one knew about, save for Natasha, an aunt to Clint's children. No previous mentions of Clint's family had been made, especially after he was missing for a long period of time under Loki's control in The Avengers.

3 THE WINTER SOLDIER KILLED TONY STARK'S PARENTS

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From the very fist Iron Man movie, we knew that Tony Stark's parents had been tragically killed in a car accident, something that had left him an orphan and in charge of his father's company. It was established as backstory, and never was a big point made out of it. It was just a fact about Tony's life. But then, all of that changed in Captain America: The Winter Soldier and Civil War.

When Arnim Zola revealed to Steve Rogers that Hydra had infiltrated S.H.I.E.L.D., he also informed him that corrections had to be made in order for their plan to work. And one of those corrections was the assassination of Howard and Maria Stark, something we would come to see had been executed by the Winter Soldier himself, Bucky Barnes.

2 THE TESSERACT IS THE SPACE STONE

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Marvel's phase one movies established the Tesseract as the ultimate weapon, an energy source of infinite power that could be used to turn the tide of the Second World War in Captain America: The First Avenger and a gateway to the other side of the universe in The Avengers. In the comic books, the Tesseract goes by another name -- the Cosmic Cube -- and it is even more powerful than that.

But when phase two of the MCU established the Infinity Stones, it was revealed that, unlike the comics, the Tesseract was not only a powerful artifact, it also had inside it (just like the gem in Loki's scepter) an Infinity Stone -- the Space Stone, to be exact. In retrospect, this revelation makes a lot of sense and it works great to tie everything together from the first Captain America film to Avengers: Infinity War.

1 STAN LEE “WORKS” FOR THE WATCHERS

For all the MCU movies there have been so far, Stan Lee has had an equal number of cameo appearances. From being a Larry King lookalike to a General in World War 2 and a womanizer on the planet Xandar, Stan Lee has appeared all over the place in different roles, no matter the era, the planet or the reality. So much so, in fact, that some fans had started speculating that maybe Stan was The Watcher, a cosmic character from the comics.

Well, the theory was so popular and convincing that director James Gunn decided to play around with that idea. In Stan Lee's Guardians of the Galaxy Vol. 2 cameo, it was revealed that while he is not technically one of these Watchers, he certainly works for them and reports to them directly. Once again, this retcon is a great new wrinkle that adds a lot more depth to the MCU.

What retcon made the most sense to you? Let us know in the comments?