Toward the end of the film, Jennings says that the place name Megiddo derives from the term Armageddon. Actually, it is the other way around - "Armageddon" is a bastardization of "Har Megiddo", which, in Hebrew, means simply "mountain of Megiddo". According to Revelation 16:16, this would be the site of the last battle in history.
When Mrs Thorne says she wants to take Damien to the wedding, Mrs Baylock says that a 5 year old wouldn't understand the goings- on at an Episcopal wedding. But since she's a Brit, and they're in England wouldn't it be Anglican? The Episcopal Church is called that in the USA. In England, it is the Anglican Church.
When Lee Remick's character, "Katherine Thorn", falls over the upstairs railing after Damien runs into the table she is standing on, Katherine's hands keep changing positions while holding onto the vertical rail posts. Her hands are next to each other in one scene, and then they are a post apart in another, and back and forth.
When the priest is physically escorted out of the building and down the steps by two guards, pedestrians can also be seen close-by. When the photographer takes a picture of the priest walking past him on the steps, and develops it, neither the security guards nor any pedestrians can be seen anywhere in the photo.
When the photographer takes a picture of the priest the second time, in the park, the picture is taken at a distance if many yards against a background of trees and tall shrubbery. The developed photograph shows the priest in close-up with no vegetation at all.
In the first darkroom scene, the photographer notices a shadow on the picture of Fr. Brennan he took with his backup camera in front of the embassy. In the background, the later picture taken on the football pitch is seen hanging up to dry. This is the one he is developing in the second darkroom scene.
Gregory Peck's hair color changes throughout the film. In most scenes it's grayish, but especially towards the end of the film (presumably these scenes were shot after the film was completed) his hair is completely dark.
In the first scene when Robert Thorne is being driven through Rome, the time is specified as 6.00AM and the date is June 6th. The sun rises in Rome at 5.30am at that time of year, which would mean it was reasonably light outside, but Robert is being driven in the dark.
The process for revealing a photo goes like this: Revealer, Water, Fixer and Water. When Jennings is revealing the picture, he takes it out of the revealer and exposes it to light, without the picture being submerged in fixer. A picture that has not been in fixer will start blackening instantly by the light. He treated the photo as though the whole process had been completed.
The engraving on the child's grave has some mistakes. The first two words are Insantem Masculum. The first word should actually be Infantem. The last line (Faecula Comtlectum" is supposed to mean "The generations embrace" but is just gibberish.
When Katherine is thrown from the hospital window, we see her crash through the roof of an ambulance. But when she is shown, she barely has any head or facial wounds at all. Plus, the blood that is running from her nose is clearly already dried.
When Mrs Thorn falls from the railing and she has landed on the floor below, Damien can be seen peering through the railings with only his head poking through them. This shot is from below. When the shot is changed his hands are now grasping the railings as he peers down.
In the final scene at the graveyard the President and his wife can be seen standing shoulder to shoulder as they are approached by the Secret Serviceman. The next camera shot, a split second later, they are standing apart as Damien has now been stood between them, holding their hands.
When the young nanny hangs herself, the window is seen breaking in three consecutive shots (outside long, inside, outside medium); it breaks in a different way in each shot (top half, bottom half, bottom 3/4).
At the end, Ambassador Thorne is shot in the church by a regular British policeman (plain clothes detective). British police are not armed (except for those on "special assignment" or those in an Armed Response Squad).
When Jennings is killed, you can see the hydraulic rubber belt flinging the plate glass off the back of the loading truck towards the manikin upon its supposed impact against the log Jennings was kneeling behind.