7. Actually, That WASN'T Korg
Marvel may actually claim this one ISN'T a retcon, because it features a character whose name is never actually spoken in Thor: The Dark World, but we absolutely know the truth. As part of the film's DVD commentary, The Dark World's director, Alan Taylor - much maligned victim of studio interference that he is - revealed that the Kronan featured in the opening action sequence is indeed intended to be Korg.
At that point, he was known to Marvel fans as part of the Planet Hulk storyline, in which he's imprisoned on Sakaar after Thor defeats him after his people attempt to take over Earth. So seeing him smashed to pieces by Thor made sense and acted as a nice little Easter Egg.
However, Korg then somehow pieced himself back together to appear in Thor: Ragnarok. And because the first Korg had never been named on screen, he was retconned to just a nameless Kronan and the "real" Korg was introduced on Sakaar.