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)
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)
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.
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 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.
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