I'd previously read about some of the historical indiscretions in 300 the graphic novel and 300 the movie, but this article contains a more complete explanation. The most notable elisions:
Most of this you remember from history class, but the article points out the philosophical deceit in the changes made in 300.
No mention is made in 300 of the fact that at the same time a vastly outnumbered fleet led by Athenians was holding off the Persians in the straits adjacent to Thermopylae, or that Athenians would soon save all of Greece by destroying the Persian fleet at Salamis. This would wreck 300's vision, in which Greek ideals are selectively embodied in their only worthy champions, the Spartans.
This movie, like Apocalypto or Passion of the Christ, seems in a special vein of rewriting of history that is not new. An artistic interpretation is a delicate balance between volumes of historical research and an iconic shorthand needed to summarize it. At what point can we criticize that shorthand for its representation of history? Do artists get a free pass to be praised for selective accuracies in their research and praised for expressive interpretation in their art, even when their interpretation defies fact?