5. Asgard's Forces Believing All Dark Elves To Be Dead - (Thor: The Dark World)
At the start of Thor: The Dark World, Odin tells the story of how his father, Bor, fought against the Dark Elves. He explains that, every 5000 years, there's a cosmic event which causes the Nine Realms to be in line and, during the battle fought by his father, Malekith, the leader of the Dark Elves, planned to release a relic from before the universe began, called the Aether, into the Nine Realms. However, the Asgardians fought off Malekith's enhanced warriors (the Kursed Ones) and Bor contained the Aether within a stone column. The Asgardians believed that they had defeated all of the Dark Elves and that they were now extinct, but Malekith, his lieutenant Algrim and some others actually escaped into suspended animation. Even during the events of Thor: The Dark World, Odin still insisted to Jane Foster that all the Dark Elves were long dead, but that turned out not to be the case and the universe was nearly destroyed as a result. Quite the consequence of a naive and incorrect assumption, I'm sure you'll agree.