10. Thinking That Algrim Was A Regular Prisoner - (Thor: The Dark World)
In Thor: The Dark World, after having captured a group of Marauders (rag-tag invaders who were essentially a group of generic thugs), Asgard played right in to Malekith's hands by taking in his henchman Algrim, who was purposely caught as a means of infiltrating Asgard by masquerading as one of them. What made matters worse was the fact that he was carrying a gem powered by pure darkness that made him a Dark Elf version of the super soldier - a Kursed One - which increased his strength to the point that he could manhandle Thor with relative ease. Upon his imprisonment in Asgard's usually inescapable dungeons, he inserted the gem in to himself and transformed in to a Kursed One, then smashed his way out of his cell very easily. He then went on a rampage, allowing the rest of Malekith's forces to invade while Asgard was in disarray and, ultimately, killed Thor's mother and Odin's wife, Frigga. How Asgard didn't realise that one of their prisoners was a completely different race to the others in his group is beyond me. Not checking prisoners thoroughly is extremely naive.