When Hitch is being shaved, sometime after the North by Northwest premiere (8 July 1959), Peggy tells him that 20th Century Fox is offering him to direct The Diary of Anne Frank. "Diary" was already finished and premiered in March 1959, so such an offer would not have been made in that same year.
Dean Martin's recording of "Ain't that a Kick in the Head" plays when Janet Leigh meets the Hitchcocks prior to the start of filming. Filming on Psycho ended on February 1, 1960. The song wasn't recorded until May 10, 1960.
When Hitch is seated in front of his TV set watching a cartoon while listening to a classical record album, the piece that is heard, which is correctly credited, is the scherzo movement from Ludwig van Beethoven's 3rd Symphony (Eroica). However the record album cover that is on top of the TV-stereo console is entitled "Beethoven Overtures."
When Hitch is finished being shaved in the barber's chair we see shaving cream residue on his cheeks. When he looks at his reflection in the hand mirror, the residue is gone, but when we see him from the front again, the residue is back.
At two occasions we see the famous Bates Mansion in the background, facing the right side of the house, opposite of how it appears in Psycho. Originally the house was constructed with only 2 walls - the left and front facade.
Portrays principal photography and Alfred Hitchcock's office as being on Paramount Studios' backlot. In reality, as Psycho was the last picture the director would make under his contract with Paramount, by the time filming began his office, as well as the sets, had moved to his new studio, Universal.
In the on-set scene in which Arbogast (Martin Balsam in the original Psycho) is about to be stabbed, Anthony Perkins' character is seen sitting nearby in drag as Norman-as-Mrs.-Bates. In reality, Anthony Perkins was not even on set this day (Alfred Hitchcock used a female "little person" as a double so audiences wouldn't be tipped off to the murderer's true identity by Perkins' height.) In fact, the only time Perkins actually appeared onscreen as Mrs. Bates was during the big-reveal climax in the fruit-cellar. Doubles and stand-ins were also used during the shower murder for reasons of convenience and deception.
The goof item below may give away important plot points.
Hitchcock's final words in the film are to the movie audience (us): "Good evening", as if closing a segment of his television show Alfred Hitchcock Presents. "Good evening" was what the real Alfred Hitchcock opened his "Presents" segments with. He always closed with "Good night."