The actor portraying Doug has two warts above his right eye. These grew larger over the extended time of filming the movie. Since the scenes weren't filmed in chronological sequence, the warts sizes change throughout the movie. Sometimes they're barely visible and at other times rather large and easily visible.
At the 12:00 mark, the junkie in the park uses a semi-automatic pistol and when firing it, repeatedly racks the slide to put another round in the chamber.
First, a semi-automatic does not need to be racked for each shot, and second, during none of those actions is a shell casing shown being ejected.
At the 29:50 mark, Father Doug Jones quotes two passages of the Bible, citing both Leviticus 24:24 and Matthew 32:6. These two Bible verses don't actually exist. Chapter 24 of the Book of Leviticus only has 23 verses, not 24. So he's quoted a non-existent verse which falls immediately after the end of Chapter 24. The Book of Matthew has only 28 chapters, so it's an obvious joke that there are now the extra chapters of 29, 30, 31, and 32 (and probably more).
During the "all out" battle at the end of the film, the blood that pours from Carol's mouth as she lays "dying" is dark brown. Showing that it's actually just chocolate syrup. Which has been used by movie makers for years. It's consistency is the same as blood and it isn't bad tasting like most fake bloods available.
In the flashback where Father Stewart recalls his serving in the U.S. Army during the Vietnam War, standing beside him is an American soldier (who's leaning against the building) is wearing civilian clothing: tan corduroy pants and black Nike sneakers. Obviously, with the movie being a "no budget" film, the extras pretty much had to supply their own costumes.
In the final flashback of Father Stewart's Vietnam War memories, he's seen walking beside a parking lot. All the cars seen around him are obviously cars made in the 2000's, and definitely not made in the 1970's when the war happened.
During Father Stewart's flashback to serving during the Vietnam War, he's inadvertently wearing his army helmet backwards throughout the entire scene.