When Marion first gets out of her car and meets the salesman at the used car dealership, a crewmember is reflected in the car door. Part way through the shot, he suddenly crouches down.
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 Lila approaches Mother in the fruit cellar, Mrs. Bates is seated in a four-legged chair. After Lila 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.
At the car dealership, the same extras (people on the sidewalks) are seen repeatedly, walking in different directions
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.
As Marion falls out of the shower, her hair is soaking wet. But in the famous still shot of her lying on the floor, her hair is relatively dry.
When Lila and Sam are walking from their motel room to the office, the reflection of a crew member can be seen in the window between rooms 1 and 2.
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.
Janet Leigh's body double is obvious when Norman is pulling Marion from the tub onto the shower curtain; the dead woman has painted toenails while Janet had clear nails during the stabbing shots.
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.
When "Mother" exits her room and charges Arbogast, she slashes downward towards his chest. But the wound is on his face when he falls backward down the stairs.
When Norman is making his way from the house to the hotel office to greet Marion, it is pouring rain outside. However, in the next shot when he's in the hotel office his suit is completely dry.
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 Norman suggests Arbogast to join him while he changes sheets, Arbogast notices Norman pause and then move past the first cabin, after which he spots a sitting figure of a lady in the window of the house, whose shadow actually appears to be standing without support.
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 so that they are dilated.)
The shadow of the camera falls on the lady examining the pesticide can in Sam's hardware store.
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.
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.
Marion is stabbed repeatedly in the torso while in the shower then presses her front against the wall before turning around and sliding down. But there is absolutely no blood left behind on the shower wall.
Lila and Sam are able to sneak into Cabin 1 just by pushing an unlocked door. A sane, calculating criminal would have locked it to avoid someone discovering evidence of Marion's murder, but Norman is not sane by any means.
In the final reveal of Norma Bates, she is sitting with her back to the camera on an ordinary four-legged chair, like a dining chair. When Lila touches Norma's shoulder, she turns as though she is on a rotating desk chair.
In the cellar, when Norman cries "I am Norman Bates!" as Sam pulls him down, his lips are not moving.
When Norman discovers Marion's body in the shower he knocks a picture of a bird off the wall. There is no hanger on the wall. Later when he is cleaning up, the hanger is visible and he hangs the picture back up.
In the shower, when Norman is stabbing Marion, his right arm gets completely wet but when he walks away, it's dry.
In the shower, 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.
When Norman drags Marion from the bathroom to wrap her in the shower curtain, she is wearing panties.
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.
Detective Arbogast phones Sam's store to tell Sam and Lila what he's found out and suspects, but they're not the ones who hired him to find Marion - Marion's boss hired him.
In the shower, Marion's hand changes from thumb down to thumb up when she grabs the shower curtain and then pulls it down.
When the police officer backs in behind Marion's car on shoulder of the highway, no tire tracks from his vehicle are visible in the soft dirt.
In the hotel, Sam sits with his arms outstretched. In the close-up of Sam, the towel has moved across to his right though he hasn't yet moved his arms.
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.
Marion is in her apartment changing her clothes after stealing the money; as the camera dollies toward the money lying on the bed, a shadow of the camera or member of the crew briefly appears on the bedspread in the lower-right portion of the frame.
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.
The piece of toast Marion eats while talking to Norman decreases in size inconsistently with her eating it.
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 Norman has pushed Marion's car into the swamp, the sound of the bubbles creates echoes. The scene was probably shot in a studio.
In the opening shots of the hotel window, the venetian blinds are bunched higher on the right and lower on the left: as the camera closes in the venetian blinds are level with the window sill.
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.
When Norman is walking through the ground floor of the house shortly before the shower scene, he grabs hold of the giant, wooden stair banisters, which wobble unnaturally.
At the beginning, in the establishing shot of the hotel window, the angle of the shadow on the window ledge changes between shots.
When Norman places the folded up newspaper containing the money in the car trunk it is noticeably thinner than when he picked it up off the night stand.
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.
In the shower, the placement of the curtain rings varies multiple times.
The title card 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 Sam and Lila first arrive at the motel, there is a shadow of a nearby tree on the side of the motel office. When Norman runs down from the house a few seconds later, the shadow is gone.
When Lila finally meets Mrs. Bates, she recoils in horror, hitting the hanging ceiling light, sending it swinging back and forth. The bulb is clearly frosted which was invented to soften the shadows it cast. But in the rest of the scene, the shadows are stark, as from an unfrosted bulb.
When Norman has to carry Mother to the basement to hide for a few days, she's full-bodied with round legs that bounce with weight each step he takes down the stairs. But in the final shot when Lila finds her, the body is a thin skeleton.
The layout of inside the Bates mansion doesn't match the exterior. For example, there are two windows on the side wall of Mrs. Bate's bedroom, yet only one central window on this same wall on the outside.
At the time of the film's production in 1959, the California Highway Patrol did not use Fords in their fleet (only Dodges with the famous D500 power plant). Also the livery of the vehicle is incorrect; only the front doors were painted white. The roof and rear doors were black. However,the CHP star on the front doors, as well as the emergency flasher and spotlight arrangement ARE correct and the uniform worn by Mort Mills is as well.
When Norman gets Marion's luggage out of her car he ignores the trunk and goes, correctly, straight for the back seat.
Marion turns on the shower and the water is instantly hot.
The calendar belonging to the Chief of Police reads '17' on December 20th.
When Marion first gets out of her car at the motel in the rain, the windshield wipers are running at a much faster rate than they were in the previous shot from inside the car through the windshield.
In the first shot of Lila and Sam waiting to hear back from Arbogast, Lila's right arm is turned away from from her body and resting on a desk. In the next shot, it is on the arm of her chair.
Fairville on the 19th and 20th of December shows no signs of Christmas.
When Marion is looking in the rear view mirror as the cop is following her, the image is flipped showing the cop driving on the right side of the car instead of the left. Looking into the mirror would still show the driver behind as driving on the left side of the car like normal.
The drive from Phoenix to Los Angeles (the presumed location of the film's fictional suburb Fairvale, California) takes roughly 5 hours 30 minutes from downtown to downtown in the 21st century. Assuming that Fairvale is closer and traffic relatively light as it would be in 1960, Marion Crane, the protagonist, would have arrived there well before dark, if not just at dusk.
Even given the time that was "lost" after being stopped by the highway patrolman, the poor weather and the fact she used a secondary highway instead of the larger US 60 (the highway link between LA and Phoenix prior to the completion of Interstate 10) Marion Crane should have arrived at her destination within 4.5-6 hours. There was no reason for her to stop at the Bates Motel.
As Marion drives with the patrolman following her, she checks her rear view mirror. He is seated on the right side of the car instead of the left.