At the end as Sharpe and Hogan watch Teressa leave, the union flag on the building is upside down (as determined by the wide white areas of the St. Andrew's Cross relative to the half width diagonals of the red St. Patrick's Cross). When Sharpe descends the steps and stands in the road, it's up the right way.
When Teresa is helping Sharpe and his men to gain access to Torrecastro, she slashes a French soldier across the stomach, killing him. A fraction of a second later we see her sword, and there isn't a drop of blood on it.
When Sharpe saves Wellesley in the opening scene, he shoots the first French Cavalryman and knocks the second off his horse. As the third approaches, he bends over to grab a pistol from the dead Frenchman, suddenly standing on dry land, even though both before and after the shot he is standing in water up to his knees.
After Teresa has helped Sharpe and his men to gain access to Torrecastro she appears to pull down a zip fastener to remove the skirt she was wearing over her trousers. A functional version of this type of fastener was not perfected until 1913 by Gideon Sundback in the U.S.A.
Teresa is in her mid-30s, yet the Major says she was waiting to marry when the French invaded only a year earlier.
In the abandoned cottage, Harper bends and reacts to Sharpe's kick to his groin a split second before Sharpe's foot actually connects, even though Harper's face is turned away looking behind him.
When on the roof of the chapel at Torrecastro, Cooper indicates a Frenchmen (who had shot at him) with the words "Left - three o'clock". This is a phrase which would only be used by someone familiar with a clock face (which Cooper may not be) and three o'clock indicates 90 degrees to the right (not left). He then actually aims slightly towards the right of the square (more like one o'clock).
Bad strategy to be avoiding a superior French force that's pursuing them, yet still lighting campfires and playing music at night.