Character mistake: When Stuart calls home from the pay phone he's talking into the wrong end of the phone.
Character mistake: When newspapers are flashed on screen to demonstrate how the dragons have destroyed the countries, one of the headlines reads "Europes' capital cities in ruins." It should read "Europe's capital cities in ruins."
Character mistake: When the Rohirrim are preparing for the Helm's Deep battle, a man is shown sharpening a sword on a grindstone. He is doing it wrong, but it is the way that most people think is the right way. The stone should be spinning so that the blade is "shaving" it, even though that seems like it would dull the edge.