Hornblower asks whether Lady Barbara is related to the Duke of Wellington. This is in 1807. Sir Arthur Wellesley was elevated to the Peerage after the Battle of Talavera and to a Dukedom in 1814 after the Army invaded France. In 1807 he was still Sir Arthur. The title of Duke of Wellington did not exist.
At one point Hornblower and his officers give the loyal toast whilst seated. While it is true that the Royal Navy do toast the King or Queen seated, this tradition only dates to the reign of William IV (1830-1837), aka the Sailor King. At the date when this film is set, RN officers would still have got to their feet to toast the King.
During the battle between the Lydia and the Natividad, on the Natividad, the railing between the upper and lower deck, where Alvarado is standing, loses and regains a drapery of ropes as the battle continues.
In the Lydia's battle with the Natividad, the main topgallant staysail (top sail, middle mast) on the Lydia vanishes early in the battle. We can tell because the sail in front of it on the foremast is shot down. Later in the battle, the Natividad fires at the rigging, and then the main topgallant staysail is shot down. Also in the later shot the top sail on the foremast briefly reappears. Then still later, both sails reappear for a shot of the Lydia sailing, and then they disappear again as the Lydia closes in to destroy the Natividad.
During the opening narration, a world map is show with "Ethiopia" in the middle of Africa (approx, where modern Zaire is located). In fact, Ethiopia occupies most of the "Horn of Africa," a peninsula on the east coast close -- it is across the Red Sea from the Arabian peninsula in Asia and several hundred miles from the position shown. No Royal Navy map would make such an error.
After the Natividad is first captured, Hornblower states he was last in his class in sword play. This is incorrect, as midshipmen in the Royal Navy did not attend a school, but rather learned on board their ships. This is also not included in the book.
At the very end as the ship leaves the harbour for England Hornblower orders the sails up (unfurled). A ship would be pulled out by rowing boats until away from the harbour else it would be thrashed in to the side of the dock by any wind strong enough to move it.
It's 1807, and when Hornblower invites his officers for dinner before they go to capture the Natividad, they toast the King while sitting down, rather than standing to do so. This privilege was only given to the Royal Navy after 1830 when William IV became King. He had been a Royal Navy officer for many years himself, and knew how cramped things were on a ship, making standing for a toast difficult. (The book makes a point of mentioning the fact that this practice was not allowed.)
When the Spanish captain boards the Lydia to announce the alliance, he is attended by only two bosun mates. This would have been a serious breach of etiquette, as he would have rated four side boys in addition to the bosuns.
Throughout the battle with the Natividad, there is no significant change in the trim or setting of the sails on either ship, despite each making numerous changes in direction. The yards, and so the sails, should have been adjusted with each change of direction. Further, at times it can be seen that the sails of the ships are actually aback (being push backwards against a mast), a highly undesirable situation which would bring the ship to a stop. In at least one scene, this happens while the sails of the other ship, which is on a similar course, are well filled with wind.
During the conference on Rear Admiral Leighton's flagship, there are four officers who are supposedly the captains of the admiral's four ships of the line. However; two wear the single epaulette of a commander or (non-post) captain. No officer with a rank of less than post captain would normally be assigned as the commanding officer of a ship of the line.
Hornblower promises Lady Barbara "that she will be home in time for the 'first robin'". He would have known that English robins, unlike American ones, are not migratory, and remain in gardens throughout the winter.
Early in the film we are told that the ship is low on drinking water. One of the sailors steals water from the water barrel and, when the bosun finds him, he merely knocks the sailor down. Under British Navy law at the time the sailor would have been flogged, receiving at least twenty-four strokes.
When Hornblower's ship, the Sutherland, is firing to "dismast" the French ships, the resulting damage is shown using closeups of the French decks. In one a huge portion of a mast falls onto the deck. But it bounces lightly, and shows that it has a smooth bottom, revealing that it is a prop.
As Hornblower's ship, the Sutherland, is entering the French harbor the French flag on the fort is lowered and immediately raised again. However, there is a perceptible pause in the two actions as can be seen by a cloud in the background that suddenly jumps forward.