The goof item below may give away important plot points.
During the film's denouement, Holmes reveals who the murderer is and offers
as proof the murderer's supposed resemblance to the portrait of Sir Hugo Baskerville. Holmes emphasizes this by standing in front of the painting and matting out the hair and beard with his hands, so the audience can appreciate the similarities of the two men. However, the right-sided semi-profile becomes a left-sided close-up when it is framed in order to match the close-up of the killer.