Continuity: Look closely at Dr. Markway as he folds his paper at the breakfast table and you can see he is holding a cigarette in his left hand. In the very next shot, Markway is holding the cigarette in his right hand instead.
Revealing mistakes: When Eleanor runs into a room in Hill House, a close-up shot shows a mirror fall off a mantle on its own. However, a wire is visible attached to the middle of the back of the mirror and going through a hole in the middle of the wall behind it. When the mirror falls, the wire goes slack as the wire feeds out of the hole in the wall, meaning the wire was held taut to hold the mirror up on the mantle until it was time to release the wire.
Errors in geography: Because the story was filmed in England, but set in the United States, Eleanor passes a house with a sign reading "To Let" instead of "For Rent".
Errors in geography: When Nell leaves the Boston garage, we can see through her car's back window that there are two English policemen standing on a street corner.
Errors in geography: When Nell is leaving the Garage in Boston, she sees a signpost that is, for the most part correct with respect to route numbers and directions for the towns indicated. However, one sign refers to "US 50" and Nell then reads aloud her directions to take "US 50 from Boston and watch for the turn-off to Route 238." US 50 is nowhere near Boston; prior to the advent of the Interstate system of highways, US Route 50 was one of the principal highways that went straight through the middle of the country from Maryland on the East coast to California on the west coast.
Revealing mistakes: In the beginning, when the tale of Hill House is being told, a woman falls down the stairs. As she is falling, safety wires connected to her arm are visible in the initial shot from behind the banister.
Revealing mistakes: The story takes place in August, in fact it is mentioned twice that it is August and yet the characters are all dressed in winter coats upon arrival. They also wear winter clothing throughout the film.