Factual error: Towards the end of the movie, two characters finally make it to a Louisiana sheriff's office. The sign out front reads, "Vierge County." There are no counties in Louisiana, only parishes.
Factual error: It is not possible to have a conjoined twin of the opposite sex. They are similar to identical twins, which are always of the same sex, in that they are formed from a single zygote. In the case of conjoined twins, the zygote fails to divide normally.
Suggested correction: There are always exceptions to the rules and, although VERY rare, it IS possible for conjoined twins to be - rather, end up being - different sexes. There are about 10 documented cases. In one case, is my understanding the conjoined twins started out as males but a genetic mutation (or something) occurred and one male lost his Y chromosome, thereby developing into a female with no ovaries. (The now-female twin would have so-called Turner Syndrome.) I don't have any sources to quote off-hand.
The male would have lost his Y chromosome after the zygote "split", but he was a monozygotic male.
Factual error: When Ed and Lorraine Warren are trying to convince the lawyer to represent Arne, there are is a copy of "Jonathan Strange and Mr. Norrel" on the shelf behind them, which was published in 2004. The film is set in 1981.
Factual error: During the briefing, Waller shows the map of the island of Corto Maltese. The map positions the fictional island in the Caribbean (it says "Mar Caribe"), but she says it is "off the coast of South America." The map does have also a set of precise coordinates in one of the middle quadrants; 45°43'36.0"S 56°32'58.2"W, which would place it far west of Argentina, surely nowhere near "Mar Caribe." (00:22:10)
Factual error: A body-sized floor-length mirror angled away from a wall would not be able to capture a mirror image of that wall or objects on or close to the wall (e.g, drapes, clock, table, etc). (00:06:16)
Factual error: James called the "Washington Board of Psychiatry" to file a complaint against Dr. Clark (Philip). Complaints (which need to be WRITTEN) against medical doctors are filed under a State's Medical Board, which would be the Washington Medical Commission in this case. Washington has a Board of Psychology and there is a Washington Psychiatric Society, but there's no "Washington Board of Psychiatry" per se (and even if there were, a complaint would still be filed with the Medical Commission).
Factual error: Despite the film being set in the 1990s, many cars made after 2010 appear in street scenes. (00:25:00 - 00:25:30)
Factual error: The crow (mechanical at least in part) perching atop the dead tree lacked some details/behaviors that would've been displayed by a real crow. Its tail should have been thinner and longer (not short and wide, although they do fan). It would have repeatedly cawed upon noticing Geary and Fenn approaching to alert family members of potential danger. It would not have straightened its head to watch the men; the dominant eye would be used to see better (monocular vision is better than binocular). (00:04:40)