Revealing mistake: During the battle at Troy, when Odysseus and Achilles are standing together surrounded by Trojan warriors, Odysseus lashes out with his sword and kills a warrior (just before Hector appears and challenges Achilles). If you look carefully you'll see that Odysseus merely swung his sword for show; it didn't connect with the Trojan warrior who fell dead.
Revealing mistake: When Odysseus sees his ruined ship at Circe's island, he collapses near it and tries to uncover it with his sword. But there is already a small hole in the sand when he collapses in it that his legs fit into. This reveals the fact that this is not their first take of this shot.