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: Early in the movie, there is a visible circular, plastic looking effect on Gideon's arm as he removes the bullet with the knife. This is more noticeable around his shoulder when he cauterizes his wound with the hot knife.
Revealing mistake: In the opening scene as the characters play a game of hurling, a farm is visible in the background, at the foot of the mountain. The farm buildings near the farmhouse are much too large and modern to have been constructed in 1920's Ireland.
Revealing mistake: There are several scenes (most notably when the group is ambushed at Mt. Suribachi) where the bayonets attached to the guns bend back and forth showing them to actually be rubber instead of metal.