Although the story takes place in the late 12th century, the armored helmets that King Henry's children play in are right out of the 15th century, the same as one might see in films about Joan of Arc, or Henry the IVth and Vth. This choice by the costumers must have been purely aesthetic because the armor of the last 100 years of medieval times was by far the most splendid visually.
In the excommunication scene, which took place in 1164, the monks are chanting the Dies Irae, a hymn said to have been composed by Thomas of Celano, who lived from 1200-1260. The hymn is also heard in the opening scene as Henry walks through the cathedral, which would have been in 1173.
The floors in the castle's great hall are clearly shown to be bare flagstone. In the 12th century floors were covered in rushes to absorb spills, garbage, and animal feces. When they got too filthy, fresh rushes were thrown down on top. Henry was known for fits of fury in which he would fall on the floor and "chew the rushes."
After Becket averts trial and climbs the stairs, he is followed by a baron who draws his sword. Becket is standing with his left side angled towards him, but after the cut to long as he leaves the room, Becket is clearly angled towards him with his right side.
Henry's court is depicted as being in England. In fact, Henry II's primary residence and capital was Chinon, nearly 300 kilometers southwest of Paris. England was only one land in Henry's empire, which included most of modern-day France.
Contrary to one of the film's central plot lines, Thomas Becket was a Norman (Thomas Bequet), not a Saxon. Jean Anouilh admitted he discovered this after having finished his play, having based it on the outdated 1825 work "The History of the Conquest of England by the Normans", by Augustin Thierry; but he decided that it made a better story the way he had written it.
The film seems to assume that Henry and Thomas speak English. The French girl Marie is clearly depicted as speaking a different language. In fact, the native language of both Becket and Henry was French, and Henry spoke no English at all.
When Henry II's men come to Canterbury to assassinate Becket, Becket is facing the altar. The long shot shows Becket walking toward them in the distance. The next shot shows Becket still facing the altar.
During the assassination scene, Brother John raises the silver cross to strike a soldier. When the camera angle changes after John is stabbed, he has no cross, though it isn't shown or heard being dropped.