Deliberate mistake: Numerous times throughout the film, when the men draw their swords from the cloth/wood/leather lined underside of their shields, which is plainly visible in many shots, the loud metallic rasping sound is dramatically heard. It is even heard in many other instances, such as when Hector removes the blade from Patroclus' neck or when Achilles takes the sword out of the sand, after Eudorus tells him of Patroclus' death. (01:47:00)
Audio problem: Before the first battle between Trojans and Greeks, we see an overhead shot of the Greek warriors walking over the hill to Troy. We hear the sounds of their shouting, clanking armor, and the heavy rhythmic sounds of organised marching upon sand, but none of the warriors are walking in formation. (01:07:35)
Continuity mistake: When the Greeks are at Troy's walls during the fight scenes the shadows cast keep changing direction. For example, when they charge Troy at night, in one shot as they run outside the city walls their shadows are cast to their right, but in the wide shot it's cast directly in front of them. (02:20:00)
Continuity mistake: The position of the three ropes that hang on the horse's head vary, between when it is first seen on the beach, when it is pulled into the city, and when it is finally placed outside Poseidon's temple. Because they are wrapped around its head, there's no way that they'd be able to move.
Continuity mistake: At the very beginning of the film when the two leaders of both armies get off their horses, in one shot, the leader with the grey hair can be seen behind the horses walking up, yet when the camera cuts to another shot, from the other leader's perspective, he is now in front of the horses. (00:03:25)
Audio problem: As Troy is burning, Andromache finds Helen and Paris, and says, "We must go!" then Helen says, "Paris! Paris!" but their lips do not match those particular words. (02:23:05)
Continuity mistake: When Ajax fights the Trojans and then Hector, after the death of Menelaus, their positions and the positions of people standing and lying around them, change back and forth in many consecutive shots. During the Hector and Patroclus duel, the Hector and Achilles duel, and the other battles, the positions of duelers, soldiers - as onlookers, fighting or lying on the ground, change drastically between shots throughout the film. (01:19:10)
Continuity mistake: During the opening scene, when Agamemnon is talking to the leader of the other army, after Agamemnon shouts for Achilles his head/body position differs between shots. (00:04:50)
Continuity mistake: When Achilles pulls the sword out of Hector, the ground area is visible as he falls to the side, directly onto patchy dry grass. Yet when Achilles goes to tie the rope around his ankles, Hector now lies entirely on dirt. (02:01:45)
Deliberate mistake: When the Greeks are sailing towards Troy they have a headwind which is especially visible when Achilles give his speech to the Myrmidons. It doesn't makes sense to use sails under those circumstances - they would have been blown backwards by the wind if they used the sails only. Even though they were rowing, they would probably just stand completely still because the rowing would just compensate for the effect on the headwind on the sailing. (00:37:35)
Factual error: Mycenae was located far inland, instead Tiryns was at that time a coast citadel city much like Mycenae. Thus the fleet of Agamemnon would have departed from there. Also Sparta never had a port.
Continuity mistake: In Thessaly, as the two armies face each other, the large branches all over the center battle ground change or disappear entirely, depending on the angle of the shot (note the cliffs on one side and the trees on the other side). (00:02:40)
Continuity mistake: A boy is sent to get Achilles, to battle what he later says is, "The biggest man I've ever seen." In the shot facing Achilles, as the boy tries to wake him, Achilles firmly grabs hold of the boy's right arm. However in the next shot facing the boy, Achilles has him by the neck to pull him closer. (00:05:35)
Factual error: Most equipment used by the Greeks, such as the large round shields and Achilles helmet, is from the Classical Period (5-4th centuries BC). At the time when the epic is set, the Greeks used small bowl-shaped helmets and light leather shields shaped like the number 8.
Other mistake: After the battle outside Troy's walls, the Trojans rout the Greeks and Hector begins to ride back towards the city. In the final shot where Hector is riding away from the camera, if you look closely, you can see Greek soldiers standing among the Trojans, still equipped with their distinctive helmets/shields. (01:23:40)
Other mistake: During the first shot of the 1000 ships, oarsmen can be seen rowing, but their oars are not disturbing the water. Yet as they approach the beach, the shot is from the same distance and now splashes can be seen.
Factual error: When the Greek leaders are lining up to offer gifts to Agamemnon, one of them is carrying a red-figure vase shaped like a submarine. Red-figure pottery (made of red clay with a black glaze, from which lines and shapes are removed to make red images) was not made until the fifth century BC.
Revealing mistake: When we fly over the beach and see all the ships with the arriving troops, some running ashore, the CGI men look like they're running on the water's surface rather than through it, as they're making no splashes.
Other mistake: After the battle in front of the gate of Troy, the field is cleaned. This includes every scrap of flesh, every drop of blood, every footprint, and even some grass is replaced as is seen when Achilles rides up on his chariot to challenge Hector. This mistake cannot be explained by saying that the Trojans are shown cleaning the battlefield because it would take weeks even months to do that.
Audio problem: When the Greeks are sacking Troy towards the end of the film, you can hear a soldier yell "Aquí" twice. Aquí is Spanish for here. Most of the movie was filmed in Mexico and used Mexicans as extras.
Answer: He was warning the other soldiers that Achilles was coming.