Continuity mistake: During the dinner and seance at Merripit House, Sir Henry's bow tie is tied "single-ended", with one flared and one pointed tip. However, when he and Watson return to Baskerville Hall later that night, his tie is tied in a regular "butterfly" fashion (both ends flared). After they chase Selden out of the house, Sir Henry's tie is once again tied single-ended.
Plot hole: Holmes asks what had frightened Sir Charles enough to make him run away from the house. But when Dr. Mortimer described the scene, he only mentioned that at one point the prints did not show heel imprints. It is an easy deduction that sir Charles had been running, but the doctor did not say anything about the direction of the footprints.