Revealing mistake: In the first major battle between the 300 Spartans and the Persians, Leonidas is slaughtering the Persians and the speed of the scene goes from slow motion to fast motion, you see very realistic CGI blood. When the blood hits the the ground it suddenly disappears. This happens many times during this long and awesome scene.
Revealing mistake: After Leonidas pushes the Persian messenger down the rather large hole he turns around to walk away. In that shot (when he is walking away) you can see that the scar on the top left corner of his chest is only a prosthetic, because it has come unglued and is lifted up.