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 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 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.
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.
Answer: It's possible that the mark didn't appear until Mrs Baylock showed up and began teaching him about his heritage. The mark of the devil is said to appear only when Satan accepts a follower, and up until Mrs Baylock appears, Damien is a normal boy. Yes, he's the devil's son, but he knows nothing of his evil nature until she starts teaching him. In fact, at one point, Robert mentions that he knew every inch of Damien's body since he raised him and never saw anything, so more than likely the mark didn't appear until Damien had areas on his body where it could be hidden more easily.
dewinela
But it's a birthmark. He had it when he was born.