When Margaret goes to Father Brennan's apartment, he must unlock a well-bolted door. The top lock (a door chain) is fitted incorrectly allowing the chain to easily slip out the hole.
When Margaret's daughter is born, you can see her umbilical cord. When Margaret saves her daughter, the daughter misses the umbilical cord and you can see a belly bottom.
The fashion used by the supporting characters, as well as the extras in the film, do not belong to the early seventies, but to the middle of that decade. Additionally, the song "Daddy Cool" by the group Boney M. is heard in a scene. This song was released in 1976 and the film is set in 1971.
It's said that ambassador to Great Britain would need a newly born kid, but in The Omen (1976), Robert Thorn is appointed as ambassador to Great Britain after adopting Damien.
Father Brennan is shown to not be a part of the cult that wants to birth the Antichrist, but in The Omen (1976), he has 666 birthmark on his thigh, which this movie shows only the children born of the Jackal carry.