The amount of molten lead pouring down on the rioters is much greater than could have been contained in the one cauldron that Quasimodo tips over.
While examining the new invention of a printing press, the king is grasping a pendant around his neck. A moment later, his hands are down.
While watching a festival, the king and an advisor are sitting about two feet from one another. A moment later, they are inches apart.
The judge adds to Quasimodo's sentence an additional hour on the pillory for insolence, but in the event he is pilloried for only one hour; also, he is not placed in an actual pillory, which immobilized the head and hands, but tied down to a platform.
The cathedral is shown as having a full flight of steps up to the front doors. Notre Dame has always been more or less level with the square (le Parvis).
The film opens in Paris during the Feast of Fools. In medieval France, the Feast of Fools took place on January 1st, yet the film isn't set in the wintertime.
At the beginning, apparently educated men are claiming that Christopher Columbus's radical claim was that the earth was round. But it was well known then that the earth was round; Columbus's claim was that it was smaller than was commonly thought. (He was wrong.)
Notre Dame Cathedral is seen with a wide square in front of it. However, during the time the movie is set (15th century) this square did not exist. The site was occupied by the Hotel Dieu, a medieval hospital which was not demolished until the 19th century.
The prologue tells us the story takes place in the 15th century during the reign of King Louis IX. Louis IX reigned from 1226-1270. However, the end credits correctly lists Harry Davenport as having played King Louis XI, who reigned during the period the story takes place, from 1461-1483.
After Quasimodo dumps the molten metal on the crowd below him, he sits on the wall with the sky in the background. Creases in the painted backdrop are clearly visible, as well as the backdrop fabric.
Quasimodo's misplaced eye never moves or blinks, because it is a prosthetic. But in the reality of the film, this deformed eye would not reasonably be expected to blink or move - because it IS deformed and misplaced.
As Quasimodo holds Esmeralda above the rejoicing crowds after saving her from the gallows, the footage is played for a few seconds and then reversed.
After Quasimodo is crowned King of the Fools, when Frollo rides up pull him away from the crowd, the makeup over Charles Laughton's right eye has fallen away showing a tiny tear.
In this story set in 15th-century France, a character wears glasses kept in place with strings that loop around the ears. That type of eyeglasses was invented in Spain during the reign of Philip II in the 16th century.
King Louis XI, a monarch of the 15th century, is addressed as His Majesty. The first monarch to receive the title of Majesty was Emperor Charles V of the Holy Roman Empire (i.e., Germany in the 16th century).
Although the story is set in 1482 during the reign of Louis XI, Notre Dame Cathedral is shown with its 19th-century additions, including statuary.
Esmeralda is tortured in a class of iron vise called the boot, or "brodequins," that is tightened around her foot by screws to crush bones. Actually, Victor Hugo's text notwithstanding, this type of boot did not exist in the 15th century. The boot was conducted by binding a close arrangement of carefully shaped wooden boards around the naked foot and calf of the victim, after which thin, sharp wooden wedges were slowly hammered into the boot, increasing the pressure until the foot bones shattered.
As Quasimodo decides to pour molten metal on the rioters, he dances around the cauldron saying "Molten metal" three times. However, his mouth never moves.
Near the end, in the king's chambers when meeting with Frollo and the Archdeacon, the boom mic shadow follows the king onto a bookstand, then when noticed, it is pulled back.