In the opening credits the narrative states that the Hundred Years War was from the 13th to the 14th century; it was in fact from the 14th to the 15th century.
Television synopses claims the story takes place near the end of the 100 Years War. In fact, 1359 was near the beginning, with 75 to 80 years left to go.
Costumers for the film took some liberties as full-plated armor was not common until 50 to 75 years later.
King Edward II reigned 1307-1327 and "Young Edward", Edward III reigned 1327-1377. As Edward II is still on the throne for this movie, the setting has to be before 1327. Toward the end of the movie the French attack Prince Edward's castle and use a cannon to knock down the castle's door. But the first time anyone used a cannon in battle was possibly at Cambrai in 1339 (if not at Cambrai, then the French were definitely the first to use cannon in battle at Crecy in 1346), when Edward II had off the throne (and Edward III on the throne) for 12 years (or 19 years). Therefore, when the French use the cannon to attack "Young Edward's" castle at the end of the movie, they are using an instrument of war that they will not, in fact, have for another decade or two.
Correction: this movie is set in 1356 based on the battles and the capture of King John II of France. Edward III is the English king featured in the movie played by Michael Horndern.
King Edward II reigned 1307-1325, and Edward III was Prince of Wales 1312-1325 when he ascended the throne as Edward III. As Edward II is still on the throne for this movie, the setting has to be before 1325, making the "young Edward" no more than 13. But Erol Flynn is portraying a man who's great accomplishments are gained as a man (and not a boy) later in his reign in the Hundred Years Wars (particularly at Crecy, in 1346, and Poitiers, in 1356).
Correction: Errol Flynn is portraying Edward, the Black Prince, 1330 to 1376, and Michael Hordern is portraying Edward III, the king. The movie is set in 1356 when King John II of France is captured.
Errol Flynn's character of "The Black Knight" is shaving with a straight razor which was invented around 300 years (ca. 1680) after the date time era of the movie.
King Edward tell his son that these are English lands. No Plantagenet King would have said that because they view themselves as French. They saw the Hundred Years Wars as their efforts to assert their rights to their lands (France), and stave off the illegitimate claims of rival claimants. Several of the Plantagenets even lived in France and ruled their French and English lands from The Aquitaine (Southwestern France) and all spoke French. Many did not even speak English, as that was the language of a defeated people (The Anglo Saxons were conquered by a French Norman Duke: William the Conqueror, who assumed lordship over England as their new king). And though by the time of this movie, the Kings were residing in London, they continued to speak French. It was not until Henry V that English became the formal language of the Royal Court, and that happened in 1417, two years after he defeated the French at Agincourt, and became the king of both England and France (a victory very short lived: by the mid 15th Century, the English had lost all of their French lands, save that of Calais). Further, several Plantagenets are buried where they lived: France, not England. So, these men did not view themselves as English. They viewed themselves as French and the English as a subordinated conquered people. And that is how Edward III would have identified himself and his lands: French.