When Norman and Toomey have their face-off in the diner, there is a kid playing a video game in the corner. We hear the sound effects, which are clearly from the game "Pac Man", but when we see the video game it's a cabinet for "Ms. Pac Man", which had completely different sound effects from the original.
When the sheriff leaves the Bates house, he and his deputy are seen walking out of the house directly onto the dirt in front. However, exterior shots of the Bates house show three steps before the dirt.
In the scene where the bloody rag is backing up the toilet, Mary grabs all of the towels to clean it up. When she then goes into Norman's mother's room and looks back into the bathroom, there are towels on the rack again.
There is a vertical window on the side wall of the front entry (to the left as you face the door) which is often clearly seen illuminated in exterior shots. However when interior scenes of the front entry are shown, there are solid walls and no windows on either side of the entry way.
When Norman is locked in the attic, the camera tilt establishes the existence of a vertical window on the side wall of the second floor window nook. During interior scenes, however the vertical window is shown on the opposite side of the nook, while the door to the attic occupies the area where the window is supposed to be.
When Norman is painting the Motel and sees Mother in the window, he drops the paint brush, in the close up his shoes are a black suede color, further shots of him running up the outside steps and interior bedroom shots show his shoes as a beige or camel color.
When Dr. Raymond asks if there have been "any more notes or phone calls," Norman has his arms outstretched, clutching the side of the motel and support beam on the porch. In the following shot when the doctor notices a woman in the attic, Norman's hands are buried deep in his pockets.
Near the middle of the movie, Mary exclaims "who?" when Sheriff Hunt states this is Warren Toomey's car, which was being extracted from the marsh at the time. She had already met Toomey at the Bate's Motel along with Norman and encountered him and his unruly behavior at the diner were she worked.
When Mary shows up at Norman's kitchen using the back door, she is holding a foil wrapped package of fudge, she offers some to Norman and states her Grandmother sent it to her. How would her Grandmother know where she was currently residing in town? Mary appeared to be transient at the moment and needed a place to stay.
Mother is depicted standing full-figure directly 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 Toomey's face is slashed by the killer's knife, the monofilament attached to the make-up effect (intended to rip the prosthetic appliance open just as the knife passes it) can be seen just above his head in the top left area of the frame.
When we first meet Actress Claudia Bryar (Emma Spool) at the diner her hair is naturally her own, yet in the final kitchen scene where Norman confronts her from behind, this is obviously a wig or padded head piece used to soften the blow of the shovel.
The goof items below may give away important plot points.
(...for Psycho) Lila Loomis continually states that Norman murdered seven people. However, by the time line of this movie the body count is only six. His victims are Mother, her lover, Marion Crane, Mr. Arbogast, and two unidentified female victims referenced at the end of the original Psycho.
In the final scene, there are two angles of Norman hitting Ms. Spool with the shovel. In the first shot - a full swing - an overhead lamp behind Norman remains perfectly still. In the second shot - the follow-through as the chair collapses, the light behind Norman is suddenly swinging.