Factual error: In one scene Lord Cornwallis' adjutant announces to Lord Cornwallis that a messenger (Benjamin Martin) has arrived. After initially dismissing the message, Lord C. pays attention on the mention of two "Great Danes" in Martin's company. The Great Dane as we know it today had many names over the centuries, but the Danish connection only became common use in the 19th century (Comte de Buffon - l'Histoire Naturelle - 1811). Until then the British would have called them Mastiffs (English or German), English Dogges, or perhaps even Boarhounds. Actually - the English Kennel Club of Britain didn't officially recognize the term/breed "Great Dane" until 1884.
Revealing mistake: In the shot where Benjamin is riding his horse to the top of the hill, the camera pans from left to right and in the distance is the British army. But if you look to the left the trees are way too huge (a bad scale). If Benjamin had ten militamen cut one down, it would have wiped out 1/4 of the British army shown.
Continuity mistake: When John Billings, a member of the militia, finds his wife and child murdered and his house burned by Tavington's Green Dragoons, he promptly shoots himself in the head. But later in the movie, at the ending battle of Cowpens, you can see him standing in the ranks, getting shot if you look closely.
Continuity mistake: When Benjamin comes down the steps out of the meeting, he walks towards his oldest son signing up. He approaches his son and his son is facing away, but they cut back to Benjamin and his son is facing him. They cut back to the son and he turns around and then back to Benjamin and it's the same shot again. His son was turned around before he turned around.
Factual error: When Benjamin Martin and his family go to Charleston, the first view we see of the city is a shot from a hilltop, looking down on the city from the north. Charleston is a port city in the "Lowcountry" of South Carolina, a broad coastal plain. Thus, the land around the city is extremely flat, and the nearest hill of that elevation is at least 80 miles inland.