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)
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.
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.