Continuity mistake: When Katherine falls off the 2nd floor and lands her right arm is pointing up, but in the next shot her right arm is pointing down.
Continuity mistake: When Katherine gets knocked off the table and over the rail, she is hanging on to two of the banisters. Her hands start sliding down. They are near the bottom, then the camera quick cuts to another angle, and she is back near the top, then her hands slide down again before she falls. (00:51:00 - 00:51:25)
Factual error: In the cemetery, the skeleton of Gregory Peck's son is far too big to be that of a newborn, which it is supposed to be.
Visible crew/equipment: When the ambassador and the photographer are in the cemetery getting attacked by the dogs there are men standing behind a tree. I'm assuming they're the trainers - they are located when the photographer is trying to hop the fence. Look in the back right hand corner - there are about two or three men standing there. (01:21:15)
Audio problem: Near the end of the movie, when the Ambassador is dragging the child up the aisle of the church, in several spots you hear the child scream but his mouth isn't open. (01:45:35)
Continuity mistake: When Mrs. Baylock pushes Kathy from the hospital window, we hear the sound of breaking glass. However, when she plunges into the ambulance, there is no glass whatsoever, nor do we see any when she is falling. (01:24:30)
Revealing mistake: When Damian knocks Lee Remick over the balcony with his tricycle, five big goldfish in a bowl get knocked over, too. But the fish don't move; they don't flap around like fish do out of water. They are obviously fake fish.
Visible crew/equipment: When Peck is falling down the stairs with the kid, you can see the back pad through his coat.
Factual error: Jennings says that the name of Megiddo derives from Armageddon and that it is south of Jerusalem. In actual fact, Megiddo is the source of the word Armageddon, not the other way around, and it is a long way north of Jerusalem. (01:26:30)
Factual error: It is highly unlikely that a British police detective making routine enquiries would be carrying a gun.
Audio problem: When the baboons are attacking the car, Damien screams, but his mouth doesn't open.
Audio problem: When Thorn and Jennings are at the apartment in Italy they are talking about Bugenhagen and Jennings says that he is an exorcist and has read about him. However, his lips continue to move even after he has finished speaking. Although the camera is on Thorn and Jennings is blurred in the background, his facial movements are still easy to see.
Continuity mistake: When Damien and his mother are at the zoo and are surrounded by baboons, they are in a small blue car. Suddenly, before they drive away, they are in a light colored station wagon.
Other mistake: Gregory Peck and David Warner are attacked in the cemetery by four large vicious dogs, who attack their throats, arms and legs, but once they escape, the only wound seen is the one on Gregory Peck who hurt his arm while climbing over the fence.
Character mistake: When the housekeeper shows the new nanny, Mrs Baylock, where to go, she says,"Round that corner to the right and it's the door facing you." So what does the nanny do? She turns left.
Factual error: In the opening scene, Gregory Peck is riding in the dark in a car in Rome on June 6th at 6:00am. The sun rises at 5:35am in Rome on June 6th.
Continuity mistake: When Thorn gets injured at the cemetery, it is in his inner left arm, but, later, at the hotel, the injury is now on the outside of his arm; the blood is not on the inner arm, which it is supposed to be.
Continuity mistake: When Gregory Peck is driving the car to see the old priest, his hands on the steering wheel are very low down, but when the camera is higher up, Peck's right hand is on top of the steering wheel.
Continuity mistake: When the Thornes go to church for a wedding, the man who opens their door is wearing a hat, but when Damian starts to act up, the man is no longer wearing his hat.
Factual error: Keith Jennings refers to an astrologers' publication which describes a star "like the star of Bethlehem" but "on the other side of the world, the European continent." Bethlehem is not on the other side of the world from Europe, it's at the same longitude as eastern Europe and less than 800 miles from the European mainland. (01:00:20)
Chosen answer: The idea wasn't entirely Hollywood's. The antichrist is sometimes said to be "born of a jackal. " Jackals are mentioned in the Bible, and some interpretations consider them a symbol of something that is "not of God." In the movie, it means that the kid was scary as hell, literally.
Hobbes