Factual errors: It is claimed that Dunwich was built on the ruins of Salem, Massachusetts. However, Salem still exists to this day.
Factual errors: The people of Dunwich are said to be descended from "Salem witch burners", although no accused "witches" were ever burned in Salem (most were hanged).
Factual errors: Mary could not have been buried alive as she would not have survived the embalming.
Revealing mistakes: Fake plastic head is visible in the extreme closeups of the intestine-vomiting scene.
Continuity: In the close-up of the priest putting the noose over his head, he would have had to have been standing on a chair, ladder or gravestone in order to reach the noose. But in the long shot, when he's hanging, there is no ladder, chair or any other item in view on which he could have been standing.
Errors made by characters (possibly deliberate errors by the filmmakers): When the Peter Bell character is talking to the policeman he refers to Sgt. Clay twice in the conversation. The first time he correctly calls him Sgt. Clay. The second time he incorrectly calls him Sgt. Bell.