When a Satanist attempts to run down Tom and Julie with Mark's station wagon, the car's front bumper is undamaged but in the next shot and before it hits a small wood pole, the bumper is bashed in at the middle despite not hitting anything.
When the Satanist tries to run over Tom and Julie in the station wagon, Tom fires a shot through the car's rear window. When Tom and Julie later drive off in the car, a close-up shows two bullet holes in the rear window, then in the next shot the window is intact.
When Julie Preston has her flashback of Jonathan Corbis during what appears to be the colonial era, Corbis makes the statement "Who amongst thee dares cast into an eternity of Limbo..?" In the context of the movie's theme, the religious references are routinely peculiar. Yet the settlers portrayed practiced a Puritan tradition, so the remark about Limbo would not reflect Puritan or most Protestant beliefs, as Limbo is primarily a Catholic concept.
In English grammar, Thou, Thee, Thine etc are singular. When addressing more than one person You and Yours are used.
There is a scene where a rocking chair moves by itself. The camera pans up to show a knife in the arm of the chair. Fishing line is clearly visible extending from the knife to somewhere off camera.
When Mark Preston leaves the house the morning after a torrential rainstorm the roads are dry and dust flies as he drives away.
Ida Lupino's wig in the first scene was clearly a mistake.
As Julie drives away from Tom before he goes back to town, you can see shadows from off-screen crew visible.
During the rainstorm in the first scene, one can clearly see the spray from the hosed water over William Shatner's shoulder.
The Preston family has been hiding the Book from the cultists for centuries yet when first Mrs. Preston and then Mark Preston are converted to the cult, no one thinks to ask them to retrieve the Book.