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: There is a scene just after when John is finally taken out of the rubble and you see the New York skyline with all the smoke in the distance. If you look closely you can see the smoke moving but the water of the river is completely stationary, indicating a still shot.
Revealing mistake: Lighting screens are reflected on Casey's chess opponent's glasses.
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: Kirsten Dunst's contact lenses can be seen multiple times during close-ups.
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.