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: When a Spartan is climbing down from the walls, he steps on a Persian body, and his foot sinks right into the corpse, showing that this "body" is a stuffed dummy. (The body is not boneless, drained of blood, or crushed by the Spartan's foot. It's a mannequin.)