Continuity mistake: In the scene when Robert Thorn arrives at the US embassy after the nanny's suicide, he gets out of the car and points at the Saudi ambassador, who can be seen in the next few shots right behind him. But in the crane-shot when Robert Thorn walks up the staircase, the Saudi ambassador and his entourage can not be seen until the last moment of the shot, when the ambassador bobs his head on the left edge of the screen.
Continuity mistake: When the guy in the car drives and hits the lamppost you can see the right side of the car is wrecked, but when he drives away it's fine.
Revealing mistake: When Robert Thorn and Keith Jennings are opening the graves, they are sitting and kneeling on the snow-covered ground. However, none of the snow clings to their clothing, and Roberts light trousers do not get any wet spots on them from the snow, revealing that the snow is fake.
Continuity mistake: When Robert Thorn and Keith Jennings are first presented with the knives, Bugenhagen lays them out. When Robert runs out Keith folds the knife belt in half, then in half again but in the next scene when he catches up to Robert the belt is rolled up.
Continuity mistake: In the end where the policeman shoots Damien's father, you see him shoot him with a handgun, nothing wrong with that, however when the shot switches back it's the same policeman but carrying a submachine gun.
Factual error: There is a reference to Meggido (aka Har Meggido aka Armageddon) as being "south of Jerusalem". Meggido is actually to the very north of Jerusalem.
Continuity mistake: In the scene where the man is dragging the manhole cover, he flicks his cigarette butt to the ground. In the shot where it leaves his hand you can see there is still white left of the cigarette. But in the next shot it's shown hitting the ground and it has magically burned right down to the filter. (00:12:15)
Continuity mistake: As Robert and the photographer are talking when they stop at the roadside coffee trailer, the camera swtiches back and forth following their dialogue. In these shots of the photographer, there is barely any snow falling behind or in front of him, compared to the shots of Robert, where snow fall is noticably heavier despite fact they are sitting right next to each other.
Continuity mistake: Right before Father Brennan receives the harpoon from above, he's banging on the doors. In the next shot when he looks up to receive the harpoon, he's at least 12-15 feet away from the doors.
Continuity mistake: In the scene where Robert Thorn talks to the man with the weird face that is wheelchair-bound, near the end of the scene, you see that the man's hood is down. Right before the end of the scene, in the frontal shot of his face, the hood is up.
Continuity mistake: In the scene when Mrs. Baylock appears for the first time, Robert and Katherine kiss and hug right before Mrs. Baylock is announced. When Katherine hugs him, she first puts her right arm around his shoulders and has both of her hands on his right shoulder, but in the next shot - when you see her from behind - her right hand lies on his left shoulder.
Factual error: The exteriors of the US embassy in London would imply that it is seated in a Thames side position, rather than Grosvenor Square, a couple of miles inland. The London Eye and Big Ben are visible from the side of the building, which is not the case in real life.
Factual error: The town of Megiddo isn't named after Armageddon, as its says in the film. Armageddon is named after the town of Megiddo.
Factual error: The filming locations in Prague look nothing like London or Rome. The most obvious mistake is in the last scene, when overhead tram cables can be seen - Central London has no trams.
Continuity mistake: When Damian is swinging on the swing and his mother turns around to answer the phone (he's not on the swing anymore) and then when she turns back around (he is on the swing) and then off the swing again when she turns around.
Factual error: When Thorn tries to kill Damien he takes him into a church at night. No church in London would be open at night unless a service was going on.
Suggested correction: Supernatural forces of good and evil have a vested interest in trying to save or eliminate Damien. Throughout the Omen trilogy it is implied that they might be on hand to subtly influence the course of events. Maybe God (or one of God's agents) unlocked the church door to help Thorn. Or maybe the Devil decided to wind up Thorn by letting him get into the church, only to make things go wrong when they got inside! but you DO lead ME on to something that puzzled ME. In the first Omen film the very young Damien is taken into a church (in point of fact Guildford Cathedral). As the son of the Devil he has a great aversion to all things Christian, so he has a huge tantrum, and screams, struggles and resists going into church. In later films he can enter religious buildings without any ill effects.
For a long time I have wondered about countering your observation on The Omen. I am 64 years old and worked for an organisation that takes care of historic churches. I also have pursued specialist historical research on churches, and visit historic churches for a hobby. I agree that, generally speaking, churches in London are locked at night unless a service or special event is taking place. Yet sometimes parish priests and church custodians can be very absent minded, and just leave a church door unlocked. Or maybe a late-night service is scheduled, so the priest leaves the door open all evening. I have gone around London and other big cities at night and passed churches, and, just for curiosity, tried the door. Sometimes I could gain access. So, all things considered, it is unlikely that a church in London would be locked at night after dark, but this is not wholly beyond possibility.