Phylon was ordered to use torches to light the hay by pretending to be dead. He was clearly given 4 torches, however, when we cross to him later, he only has 2 torches.
Early in the film, the Persian King threatens to cut off the head of a man. As the scenes cut back and forth, we see the chopping block in heavy shade, then in bright sunlight, then back in the shade.
As the Immortals are charging the water behind them is choppy, when it cuts to the waiting Spartans the water is calm. The water action does not match until the two lines are together.
Several times the Greeks refer to "Jove". Jove or Jupiter was the chief god of Roman mythology. His Greek counterpart was Zeus.
When the Persian army is into Greece where they will meet the Spartans, they are marching between mountains on their left, and water on their right. In actuality, the Persians were marching south along the eastern edge of Greece, so the water would have been on their left with mountains on their right. The way the movie portrays it, the Persians are marching out of Greece.
A character is clearly heard saying that the other army "runs like a machine." How would they know how a machine runs before the term machine has been commonly used? Greeks actually were familiar with the concept of machines in their plays. Plots would be resolved by a god being lowered or raised to the stage by machine, referred to in Latin as a "Deus ex machina"
When the Immortals attack, the javelins thrown by the Greeks have no points. Many of the Immortals can be seen dropping their weapons to grab the javelins and hold them under their arms or to their chests. A javelin with a blunt end can also be plainly seen hitting one of the extras in the mouth.
During the battle with the immortals, one of the Spartan "officers" is revealed to have a pink towel between his forearm and the shield itself to comfort and protect the forearm. This is revealed a couple of times during the battles.
When the Spartan soldiers are in the water in that scene, "Night Attack", the ocean and the lower parts of the hills that are in the background are dark because its obviously "night". But in the background the daylight can clearly be seen. The lower half of the land is dark and the top half is illuminated. It's not the shadow of cloud, it's not supposed to be because it's night.
While most of the Spartans wear the correct headgear, Leonidas and his officers wear Roman legionnaire style helmets that wouldn't be around for about 200 years. (This may have been so that they could easily be identified.)
Rigid stirrups are shown in use by horse riders; however, this type of stirrup wasn't in use until some centuries later. The first dependable representation of paired stirrups dates to around 322 A.D.
When a weakened Ellas is being carried into the goat herder's home by Ephialtes, you can clearly see that she is wearing very non-Spartanlike sheer black knickers.
Near the start of the film a captured Spartan, gives a stirring speech to the Persian king about Persian slavery verses Greek freedom, which is odd given slavery was common across the Greek world. At around the time the the movie is set, Sparta had around 8000 free male citizens, and around 100,000 slaves and semi slaves.
(at around 1h 30 mins) As the Immortals are marching along the old goat track, the shadow of a crew member can be seen in the bushes, bottom left of the screen.