As Leonidas is considering the Persian Messenger's offer the camera zooms in on a mother standing with her daughter, clearly visible is the curved outline of a modern brassiere cup under the woman's peplos (or chiton).
Among the weapons Xerxes' army uses on the Spartans are small bombs, which would have used explosive powder. Black powder was not invented until the 9th century A.D., and not known to Arabs until the mid-thirteenth century.
Several characters have vaccination scars. A very noticeable example is Daxos' left bicep when he and Stelios are looking at the amassed Persian forces and talking about the Spartan's wish for a beautiful death.
When Leonidas asks his 300 what their profession is on the hill, his booming voice leaves a very short echo, indicating obviously that the scene was filmed inside of a small studio as opposed to the open mountain terrain shown in the image.
After the Spartans are attacked with the shields for the first time Leonidas stands and breaks the arrows off leaving the heads in his shield. When he turns to attack moments later the arrows are gone completely.
During the ending of the fight with the immortals, Leonidas' helmet has a coat of hair on top (probably to mark his status as the king). In the next scene, while he kills the immortals, he is wearing an ordinary helmet.
When the Persians first fire their volley of arrows at the Spartans, their capes become pinned down by many of them as they duck, but when they stand up, there are no arrows in their capes, nor any corresponding holes.
When Queen Gorgo and the Loyalist meet for the first time, they speak against a backdrop of torches. We see their shadows as they talk on the sand. However, when we see them speaking again, the floor beneath them is all stone. It is not possible that their shadows could have reached all the way from where they speaking, down the steps, and out into the sandy courtyard.
After Leonidas' face is cut by the immortals giant, throughout the movie the cut moves slightly (e.g. at first the part under his eye is slightly slanted, then later on in the movie, the cut starts farther down on his face, and it is more straight).
The first time the Spartans are attacked, a Spartan's arm is grazed with a spear over the top of the shield wall. When the spear moves forward it cuts the arm, then in the next shot from another angle, the arm has no wound yet and gets cut by the spear retracting.
During the final scenes of the movie, when the camera fades out on to the vast army, Dilios does not have his helmet and the spear is in his right hand, but in the next scene the spear is in his left hand and he goes on to wear the helmet using his right hand.
When Leonidas is carrying the child out of the destroyed village, the child's head is hanging below Leonidas's arm. However, when Leonidas stands in front of the Tree of the Dead, the child's head is level with his arm, and no longer drooping.
Xerxes, or any Persian ruler, would never have referred to himself as a god or a god-king as an Egyptian Pharaoh would. It would be the highest blasphemy. Their title was roughly translated as King-Emperor which the British Monarch used during the height of their Empire. The Persians, specially the Persian kings, were Zoroastrians, who were monotheists with a single supreme Creator. Their god is considered, by many, to be the same as the Judeo-Christian-Islamic God.
The final test for a Spartan warrior was not the wilderness survival, though that was a part. The final test was to sneak out of the barracks, kill an unsuspecting slave, and return without being caught. The wolf story of Leonidas might actually be loosely based on Xerxes. To prove he was worthy to be King, Xerxes was locked in a room with a full grown lion, armed only with a spear.
The Greek forces at the three-day defense of Thermopylae were not only 300, nor were the Persians over 1,000,000. There were at least 4000 Greeks involved on the first two days and another 1500 men involved in the fatal last day stand. Furthermore modern scholars rejecting the figures of the Persian army given by Herodotus and other ancient sources as unrealistic, estimate the number of Persians in the range 70,000-300,000.
Despite Leonidas correctly explaining to Ephialtes that the Spartan strength comes from fighting in a Phalanx, through out the movie they rarely fight this way. Instead, the fight scenes are more traditional and more exciting.
During the battle scenes, the blood never appears on the ground. In one scene, the blood hits the ground and disappears; in many it vanishes in the air. In another, the droplets fall and stick out of the ground like arrows. This is a stylistic choice to reflect the 'graphic novel' origins of the film.
In the scene with the oracle where she is floating/dancing you can clearly see her holding her breath (her cheeks are puffed out). This was due to the scene being filmed underwater and then removing the water.
The sun sometimes changes position in the sky, most noticeably when the Spartans push the Persians off the cliff. In this shot, they are silhouetted against the sun, which is level with them, but seconds later the distant Persians fire arrows which blot out the sun now high above them.
Whenever you see an arrow being shot they have barbed or wide arrowheads yet after they have hit someone the are always in a small round hole. The arrows would look like they are sticking out of a cut the width of the arrowhead.
The goof items below may give away important plot points.
In the last fight scene, the Persian archers are all lined up behind the Spartans, but when Xerxes gives the call to attack, the arrows come from in front of the Spartans and hit them in their chests rather than their backs.
Dilios is telling the story to the new soldiers he is commanding before battle. Yet he leaves before King Leonidas and his men execute their final attack. So he wouldn't have known what they did exactly but he is telling the men everything until the point of Leonidas's death as if he was still there for it. However, the character of Dilios, according to director Zack Snyder in the documentary The 300: Fact or Fiction? on the 2-Disc Special Edition DVD, is not a man to allow truth get in the way of a good story and is narrating in an effort to invigorate his men before a later battle.