Deliberate mistake: During the Trojan attack on the Greeks at the beach, before Hector and "Achilles" have at it, there are two separate close-ups of Hector's sword slicing across a man's face, who promptly spins around and spews blood. This particular man is enjoyed twice in two takes of the same shot, just before and after Odysseus' close-ups. Then as if that's not enough, a few shots later this spewing man is actually seen a third time, but this time from another angle. (01:42:30)
Deliberate mistake: When Achilles reaches the bottom of the stairs of Apollo's temple a Trojan pulls on Achilles' spear so he elbows that Trojan in the face, then turns and slices another one. When Achilles turns around he is face to face with a particular Trojan, who has two decorative clasps in his long hair, then he slices him across his face and he falls. In the next shot he fights another couple of Trojans. The following shot is the exact same shot as the previous one with the Trojan, who has two decorative clasps in his hair, that he proceeds to slice across the face again. (00:42:55)
Deliberate mistake: Stable gates never open inwards. In two scenes after Achilles leaves the solder alive during the ravage of Troy, they do. This is probably because it is visually effective when soldiers kick gates from the outside to open them. Stable gates in all cultures open outwards so that in case of fire the animals can't block them. (02:22:05)
Deliberate mistake: During the battle scenes at the gates of Troy, the bodies that lie on the ground are rubber or something similar; people trample over them and they bounce. (01:21:25)
Deliberate mistake: Looking from the left side of the wooden horse at the temple which is being plundered and burnt, a soldier is stabbing a citizen in white robes in the middle of the steps. A second soldier chases another citizen in white past him on the right. Then moments later the same shot is seen from the right side of the wooden horse. Obviously, two cameras filming the same scene. (02:22:30 - 02:23:35)
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)
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)
Answer: They didn't know the Greeks weren't bringing cavalry. The Greeks did use them and sometimes brought them by sea. It was there to stop a cavalry charge and to break up fighting formations.
LorgSkyegon