When Tom Dobbs (Al Pacino) joins the army, he is greeted with "welcome to the United States Army". The army was actually known as the Continental Army at that time as Washington and his revolutionaries had no way of knowing that Canada would fail to be won in the War Of Independence.
In the battle scene, a caplock musket is visible. The first caplock firing mechanism was not patented until the early 1800s, 20 years after the end of the American Revolution. Caplock firing mechanisms depicted in the film were not in wide use until the 1850s, 70 years after the American Revolution.
Despite having his eyes closed, Ned Dobb screams with agony nearly a second before the Huron even touches his poisoned foot with the heated arrow head, at a moment when it is still at least three inches away from it.