Continuity mistake: At the battle of Edgehill when Crowell's cavalry and Prince Rupert's cavalry are in the middle of a melee and you can see a wound on Crowell's left arm, the same wound he receives at Naseby. It is clear that the scene was meant to be used for the battle of Naseby but was cut and used at Edgehill.
Continuity mistake: During the attack a parked US plane is blown up and the prop goes cartwheeling off along the runway. Not wishing to waste a perfectly good explosion the director filmed this from several different angles and used the same explosion several times. You can easily see the prop each time.
Continuity mistake: During the entire battle the ground is sticky mud. When the French cavalry charge however the ground is dry, so dry there is dust coming from the horses' hooves as they gallop.
Continuity mistake: When the two different brothers retrieve the book from the Narrator (and ultimately shoot or get shot), in almost every shot, you can tell the scenes were shot separately by looking at the background. Donald Sutherland's shots are different by the wind hitting the plants in the background; Gene Wilder's shots are different by the position of the sunlight (i.e. one is dark, the other is fully lighted). Obvious if you look closely.