Continuity mistake: During the start of the battle in Odysseus's throne room at the end of the film, watch as the evil suitor hurls an axe at Odysseus and Telemachus, shouting "A river of blood!" Both duck as the axe flies towards them. Then in the very next shot, they duck a second time as the axe smashes into the wooden throne behind.
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: The gaps between the boards of the Trojan horse are much wider in the inside than on the outside. This reveals that the soldiers are not really inside the horse, but on a set.