When the French king is preparing to land on the English Coast, he is told that they would land in "about 40 minutes". Time was not measured in minutes at that time. The closest they could tell was before mid-day or after mid-day.
The stone castles are of a style not seen until the reign of John's grandson, Edward Longshanks, and England was not known for its longbow archers until after the time of Edward as well. England got its longbowman from Wales as mercenaries until after the time of Wales' final defeat during the reign of Longshanks.
Marian, leading the plow horse 'Goliath', appears to help plow the field with a moldboard plow (a curved board that turns over the slice of earth cut by the share), which was not introduced into England until the 18th Century.
When reading the note carried by the pigeon, Marshal is shown using a huge magnifying glass, or lens. Although magnifying glasses have been described in England in the 13th century, they wouldn't have been of that size and quality.
When Robin and his men are on the ship from France to England, they are drinking out of glass goblets. Glass was very expensive, and would never have been used on a ship. Instead, they've have had pewter or wooden drinking vessels.
After Isabella of Angoulême tells John that the French king Philip is approaching England with an army she holds the point of a knife to her chest which is then dropped to the floor. As it hits the floor it is clearly shown to be a distinctive Indian dagger known as a "Pesh-Kabz" and dating to the late 19th Century. A close inspection shows that the handle appears to be made of green jadeite and mother of pearl, a type made primarily for the European market by Indian craftsmen.
All French and English soldiers are shown to wear chain-mail, a fantastically expensive excess only affordable by nobility at the time. Not that, here, it makes any difference as swords and arrows seem undaunted by any kind of protection.
In one scene, a hurdy gurdy is heard and shown being played[badly]. At this time, the instrument was in a form called an organistrum, about six feet long, played by two people and played only in churches to accompany song. The portative version is first seen about 1400.
Near the beginning of the movie when Marian is helping the men plow the field, a thunderstorm is approaching behind her (as evidenced by the dark clouds and thunder). Yet a few minutes later when the sheriff begins harassing her, we only see puffy white clouds and blue sky.
When Robin first discovers the inscription on the sword hilt in the boat, the inscription reads from the base of the blade to the pommel. But when he re-reads the inscription by the fireplace on his first night in Nottingham, it reads from the pommel to the base of the blade.
Robin and the troops arriving from Nottingham meet up with William Marshal and his troops at the White Horse in Uffington. This is northwest of London, approximately 160 miles (by modern driving routes) from Dungeness, where the French troops are landing. It's impossible that they would have arrived in the time depicted.
At the meeting in Barnsdale, William Marshal informs King John and the Barons that the French are "in the channel". Then, after the battle at Nottingham, the French officer informs Robin that the French will land in two days. (aside from him having no way to know this since the message to Philip to invade had just been dispatched by Godfrey when they marched on Nottingham). The channel crossing from Calais to Dungeness is only about 25 miles and would not take two days.
In the Director's cut of the film on DVD, the French land in "Dungeness" (in the theatrical version it is never identified by name). Dungeness is a flat marshy area with no cliffs as depicted in the film. The area looks more like Dover, which would have been a more likely choice to invade since it is somewhat closer to France than Dungeness, although not considerably so.
In the establishing shot of King Richard's campsite in France, a burnt village can clearly be seen on the right side of the frame, yet it isn't mentioned. Later in the movie, we see the same exact village, but this time it's Barnsdale, England, where Robin is from. The village is unharmed, but soon it's burned by Godfrey and his men, revealing to us that it's the same set from the one in France.
The real Isabella of Angoulême was not the niece of King Philip II of France. She was the only daughter and heir of Aymer Taillefer, Count of Angoulême, and 12 years old at the time of her marriage to King John. Isabella was originally betrothed to Hugh IX le Brun, Count of Lusignan, son of the then Count of La Marche. As a result of John's temerity in taking her as his second wife, King Philip II of France confiscated all of their French lands, and armed conflict ensued. Unlike in the film, Eleanor of Aquitaine readily accepted her as John's wife. Ironically, after John's death, she married Hugh X of Lusignan, the son of her original fiancé.
Richard states he can't wait to lock his mother and brother up. In fact, Richard and his mother were quite close. She had been imprisoned by her husband, Henry - and one of Richard's first acts as King was to order her released.
Marian uses a longbow with no apparent effort. It took a large, strong man to draw a longbow. Estimates on the draw-strength of a C13 medieval English longbow are between 80 to 110 pounds. The pressure involved in her drawing the bow would be nearly the same as lifting her own weight mainly using normally weak muscles like the triceps.
Though there are primary sources dating back to within a few years of when the film takes place which describe landing craft that "had doors, which were easily opened, and a bridge was thrust out whereby the knights could come forth to land all mounted", these doors were in the stern of the ship, not the bow, and were not intended for amphibious landings. [William of Tyre, Chronicon, ed. Robert B. C. Huygens (Turnhout: Brepols, 1986), p. 927]
Isabella of Angouleme is shown speaking French at various times throughout the film, while others at the royal court speak English. However, the court language at the time was French and none of the Angevin kings spoke little, if any English. The first king likely to speak fluent English is John's grandson, Edward I and no king spoke it natively until Henry IV.
Several characters speak of "seed corn." Many viewers interpret this as American maize which wasn't introduced to England until the 15th or 16th century. However, the word "corn" in 1199 England was used for many different cereal grains (wheat, rye, oats, etc.), not the corn-on the-cob we think of today.
The shields carried by the rank and file troops do in fact have uniform indentations in them. However, these are not meant to be battle damage. Medieval shields were constructed of thin sheets of wood glued together in cross-grain layers - literally, plywood. To increase durability, they were covered on the front with fabric or leather, sometimes with a a thin layer of padding in between, and then painted. The "holes" that have been pointed out are in fact where the rivets that attach the arm and shoulder straps (when not in use, the shield was slung over the back for easier carry) have indented the covering material. This is what gives the uniform appearance.
From 1066 until 1399, English kings spoke French in their daily lives, and Latin in some diplomatic transactions. They usually did not even learn to speak English, which they regarded as a peasant language beneath their dignity. Their speaking English in the film is an acceptable artistic decision, consistent with all English and French characters speaking in modern, rather than medieval, standards of language.
It has been said that the size of Robin's childhood hand mark on the carved stone when we see Robin's memory fits the size of his hand as an adult. However, Robin places his hand in the hand mark of his father which fits. When the marks are made, Robin (as a child) makes the mark on the left, while his father makes the mark on the right. It's the mark on the right that the adult Robin places his hand into.
In the beginning of the movie, an army of 3000+ men is left to pillage and plunder in France after Richard dies; nothing is ever made mention again of these men in the movie. Yet the French are able to raise an army to invade England while leaving this other British army behind on French land. Where did this French army come from to be able to invade England?
Cremation, as done to Sir Walter Loxley, was frowned upon by the church and therefore was forbidden in parts of medieval Europe (mostly those where the church had a strong influence, England being such a place) other than in the case of mass cremations for disease and as a punishment for heretics. The characters acknowledge this in a deleted scene: Walter tells Marian of his wishes to be be cremated. She responds by joking with him about being a good Christian, and if he doesn't stop with the subject she will tell on him for being a pagan.