Messenger: What makes this woman think she can speak among men?
Queen Gorgo: Because only Spartan women give birth to real men.
When Leonidas, and presumably all the other Spartans, use their spears to clear their shields of arrows after the arrow shower from Persian archers, the tips of the arrows are still visibly left on all their shields. In the next shot, when the Spartans regroup for the Persian cavalry charge, the arrow tips are gone. (Granted, this is a highly stylized movie based on a comic book, but that does not change the fact that this is a continuity error.)
Some weapons used by 300 are actually weapons from previous war epics like "Alexander" and "Troy." They were used in this film to cut costs.