10 Movies That Made Subtle Retcons They Hoped You Wouldn't Notice
2. The Tesseract Was An Infinity Stone All Along - Thor: The Dark World
As much as the Marvel Cinematic Universe has successfully created an interconnected world of movies with the utmost care and attention, it goes without saying that not everything was laid out in advance.
We can go back to Phase One to see proof perfect of this. Captain America: The First Avenger was the first film to introduce the Tesseract in earnest, which was very clearly based on the Cosmic Cube from the comics.
In the source material the Cosmic Cube was capable of controlling matter and energy, but once Kevin Feige and company decided to go the Infinity Stones route, they retconned the Tesseract to be an Infinity Stone, in turn separating it from the Cosmic Cube entirely (which in the comics was categorically not an Infinity Gem).
The retcon took place in Thor: The Dark World's mid-credits scene, where Volstagg (Ray Stevenson) identifies the Tesseract as an Infinity Stone.
Granted, the whole Infinity Saga was in its absolute infancy at this stage, so the retcon was far easier to get away with than had Marvel waited a few more movies to do it.
And for most casual MCU viewers, it went down totally smooth, so there's not much of an issue here.