When Norman first meets Marion his first words to her were an apology for not hearing her on account of the rain. He then asks her to accompany him into the office. His lips don't even move during this scene and he gestures with a hand signal for her to go inside instead; the audio must have been added later in post-production.
When Marion drives away from the police officer, the unmistakable sound of a 1957 Ford starter can be heard, but she doesn't reach for the key (which is left of the steering wheel on the dashboard), or make any visible movement to use the shift lever.
While driving in Bakersfield, no car dealership can be seen through the windshield as Marion turns off a major road onto a wide, unlined street. The next exterior shot shows her car turning off a four-lane, lined street into California Charlie's used car lot.
As Marion approaches the restroom at California Charlie's used car lot, the door hinges on the left with the doorknob on the right and has dark (painted?) glass in the upper half. In the interior shot, the hinges and knob are just the opposite and the door is a solid slab. In the exterior shot as Marion exits the restroom, the door is not seen, and hinges are back on the left and latches on the right, consistent with the original view.
When the cop awakens Marion in her car, the driver's side windshield frame has dozens of grimy handprints and fingerprints from everyone whose touched it while setting up the shot. As she pulls into the car lot the grimy hand prints / smears are shown to be all over the door panels also.
Det. Arbogast phones in about the Bates Motel and Norman. Later, he returns to the motel to investigate. There is a reaction shot of him looking at the Bates house. The sky in the background is clear and uniform. Arbogast glances behind him to make sure he isn't shadowed and then starts out for the house. Now, the sky in the background is obviously cloudy.
The captions at the start of the film place the date as December 11th. Later in the film, Lila tells the sheriff that Marion disappeared "a week ago yesterday." This makes the date December 19th. But the calendar in the police station as the psychiatrist gives his report says it is the 17th.
When Sam and Lila are checking her motel room for traces of Marion, the lid over the toilet seat is up in the bathroom when they enter it, but when Norman left the bathroom after finishing cleaning it up, it was down.
When Marion pulls into the motel during the rain and sees the office, she drives over to it and stops. In the next shot two lights on a stand can be seen to the immediate left of the office, left by the crew.
The title card of the film fixes the date as December 11, but when Marion is deducting the cost of the car from the $40,000 later that same night, the last date in her bank book is shown as being January 20. People often "backdate" checks.
When Lila approaches Mother in the fruit cellar, we see Mrs. Bates seated in a four-legged chair. After Ms. Miles touches the corpse, it slowly spins around as if it's sitting on a swiveling chair. The effect was achieved by a prop man lying on his back rotating a camera head with wheels underneath Mother.
When Janet Leigh is in the car dealer bathroom getting the cash, as the envelope is being returned to her purse the top couple bills fold back revealing a $1 bill, not another $100 as the stack is expected to contain.
When Marion Crane leaves town and it becomes dark outside, the rear shot of the car driving into a cloudy night shows very noticeable white scratches in the sky. Apparently, a scratched image of the dark cloudy sky was placed above the footage of the car driving to make it appear as though the car was driving in the night.
During the shower scene, when Marion is seen in front of the shower curtain, there are two different water jets, visible by different angles towards each other, revealing that besides the shower head, an additional source of water was used.
The goof items below may give away important plot points.
When Norman comes to clean the evidence of Marion Crane's death, there is blood on the floor. In the immediate capture, there is seen less blood in the same spot. Norman couldn't have mopped the blood away that quickly.
There are persistent reports that Marion swallows after she is dead. The story originated in a newspaper article in 1973, but has been misremembered and misreported by subsequent generations of goof fans. The original story said that Alfred Hitchcock's wife, Alma Reville, spotted the post-death breath shortly before the film was released and told her husband in time for a correction to be made. According to Janet Leigh, it wasn't a breath at all, but a blink, and it was, indeed, edited out. However, watching the shower scene on the Collectors Edition (ISBN 0783225849) appears to confirm that while Leigh does not actually blink, there is a very slight twitch of the eye that can be spotted by watching the reflection of light. Also, Leigh does indeed appear to have a contraction in the throat at the very beginning of the shot, visible in the upper left of the frame; not an obvious gulp or swallow as has been reported.
Lila and Sam are able to sneak into Cabin 1 just by pushing an unlocked door. A sane, caluclating criminal would have locked it to avoid someone discovering evidence of Marion's murder, but Norman is not sane by any means.
Mother is depicted standing full-figure in front of the second-floor front window. However, the arrangement of the furniture in Mother's room establishes that there is a large desk in front of the forward-facing window, which would prevent a full-figure glimpse of anyone at that window.
When Marion is lying dead in the shower, an extreme close-up of her eyes is shown, which shows her pupils to be narrowly constricted. A dead person's eyes are fixed and dilated. (There is probably nothing Janet Leigh could have done about this, especially since the bright lights used in filming would cause her pupils to naturally constrict. However, the extreme close-up makes this point particularly noticeable, and Gus Van Sant made it a point to digitally alter Anne Heche's eyes in Psycho (1998) so that they are dilated.)