The film is billed as "Edgar Allan Poe's The Haunted Palace." While Poe did write "The Haunted Palace," the film is actually based on the story by H.P. Lovecraft, "The Case of Charles Dexter Ward."
When there is a shot through the fireplace, the evenly spaced flames reveal that a flame bar is being used in front of the lens.
Obvious dolls when characters fall into fireplace and are burned to death.
The main door of the castle has the deadbolt on the outside, possible the doors were installed the wrong way around because the opening handles are on the inside.
In the funeral scene you can see folds on the fabric background.
About halfway through the movie, when the painting of Curwen is speaking to Ward; as the camera zooms in on Ward you can see the shadow of some equipment visible on the bottom right of screen.