Revealing mistake: This scene takes place just outside the Thorn mansion in a snowy wooded area. Mark confronts Damien about being the Antichrist. Damien says yes and to join him. Mark declines, and then drops dead. In an extreme close-up, Damien yells out "Nooo." If you pause on his close-up, you can see dental work. Fillings or caps, whatever it is. He is supposed to be the Devil's son. He has never been sick or injured (mentioned in The Omen), and is immune to the chemical accident in this film, but oral bacteria? Secondly, even if his teeth can be vulnerable like ours, Damien was written as a 13 year old boy. That's a bit early for that kind of dental work.
Revealing mistake: When Joan Hart is knocked down by the truck she is replaced by an obvious dummy, it is more obvious when the dummy hits the ground.