Revealing mistakes: 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.
Anachronisms: 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.)
Anachronisms: Spartan physician states that spear has missed an artery. Many centuries early for circulatory system knowledge. Never the less, an ancient physician would have known that cutting an artery (or whatever the ancient Greeks would have called such a blood vessel) would have been a very bad thing for the recipient.
Miscellaneous: 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.
Incorrectly regarded as goofs: 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"
Factual errors: Several times the Greeks refer to "Jove". Jove or Jupiter was the chief god of Roman mythology. His Greek counterpart was Zeus.
Errors in geography: 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.
Revealing mistakes: During the battle with the immortals, one of the Spartan "officers" is revealed to have a pink towel between their forearm and the shield itself to comfort and protect the forearm. This is revealed a couple of times during the battles.
Continuity: 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.
Continuity: 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.
Crew or equipment visible: (At 1:34:15) 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.