When Tony dials the first phone call in the movie, it's clear from the sound and his finger movements that the fourth digit is smaller than the third, perhaps a 4. But from the immediately following dialogue, the number should be HAMpstead 7899, i.e. 426-7899.
When Swann arrives at Wendice's flat, Wendice tells him they went to the same college but that Swann wouldn't recognize him because he (Wendice) had only arrived in Swann's last year. But immediately afterward Wendice takes the reunion dinner photo from his wall, showing Swann sitting right beside Wendice. Leaving aside the matter of why Swann and Wendice would be attending the same reunion since they had graduated years apart, Swann never contradicts Wendice's assertion that they hadn't seen one another in the 20 years since college.
(at around 24 mins) Wendice throws a £100 bundle on a pink armchair. The money falls right at the back of the seat. A few minutes later, Swann takes the money which is now right in front of the armchair.
Wendice shows the Inspector the photo of him and Swann at the reunion. You can see the empty spot on the wall where the photo was. After some conversation between the Inspector and Margo, the photo is back on the wall.
When Swann is outside the apartment he checks his wristwatch and it shows the time is 10:53. The dial is plainly visible and has no brand logo. Once Swann has entered the apartment, he checks his watch again. The dial is visible and shows a time of 10:58. Partially hidden below the minute hand now is a miniature of a European steam locomotive.
After Tony removes the reunion photograph from the wall to show it to Swann, he re-hangs it tilted relative to the other pictures next to it. In the next shot Tony is returning to his chair and the picture has been straightened.
In the opening scene, the big trees at the end of the street are completely bare, as in winter. In all other scenes, the same trees are full of leaves, as in summer. Yet the murder occurred on September 26th, so all subsequent scenes ought to show fall colors.
The Chief Inspector's view of the street from the bedroom changes between scenes. When Margot arrives, a side street can be seen on the left, and the red telephone box in the distance is to the right of the gate post in the foreground. When Tony arrives just a few minutes later, the side street is no longer visible and the phone box is now to the left of the gate post. The parked vehicles have all changed too. When Tony starts to leave for the second time, the perspective and light conditions change back to that of Margot's arrival, with the same cars parked on the right.
During the murder scene when Margot is stretched out across the desk with Swann laying on top of her. Margot stretches for the scissors, just as she is a about to plunge them into Swann's back. Just a little bit of the dummy scissors previously positioned in Swann's back can be seen.
When Chief Inspector Hubbard uses his penlight to illuminate the telephone dial (and later to show the lock on the apartment door) the area of light is shaped and moves like an off-screen spotlight (which it is).
The inspector makes a phone call from the flat, supposedly in London. We hear a clearly American-accented voice on the other end say "operator". It should have been an English voice, saying (in the 1950s), "Number, please."
FLOOR PLAN doesn't match script. The kitchen is clearly visible from the camera angle. It would create an L shaped apartment, going under the stairs, yet the officers are told to go out back which would slam them into the kitchen wall. Also there are no bars on the window as the script mentions, assuming they would be on the outside wall (which can be seen).
After the Inspector switches raincoats without Tony's knowledge, as Tony leaves his apartment, he pulled the door shut causing the door latch to lock the door without the need of a key from the outside.
As Wendice walks out the front door after trying unsuccessfully to get into his flat, Hubbard describes to Margot and Mark what he's doing. Wendice at first gives up on why his key doesn't work and is shown turning away from the entrance and starting to walk off. The camera then cuts to Hubbard, who says, "It's no good, he's going away down the street." But then Hubbard says "Oh, just a minute", and as the camera cuts back to Wendice he's barely moved. Given the time that has elapsed while Hubbard was speaking, Wendice should be several feet away by then.
When Inspector Hubbard is looking out the bedroom window at Margot and the policemen walking toward the apartment, their legs suddenly disappear from view, revealing where the projected scene of the street meets the edge of the physical set.
The photograph of the college class reunion has obviously been doctored. In the (doctored) photograph, it is obvious that the heads of Ray Milland, Anthony Dawson, and Alfred Hitchcock have simply been "cut and pasted" onto the bodies of three other men in the original (undoctored) photograph.
The goof items below may give away important plot points.
(at around 41 mins) Just before Margot is strangled she answers the phone but there is no one speaking. We clearly HEAR that she is hitting the phone button several times with her hidden hand. As we saw previously that this is not an 'operator' call, this should disconnect. However, the husband keeps talking, and can be heard even before he hits the 'connect' button.
When Margot has her breakdown in Mark's arms nearer the end of the film, the carefully set waves at the front of her hair fall out of place and become loose - yet in the next again scene the waves have been completely re-set and smoothed down, something she neither would have had the time to do without curlers and a brush.
A simple inspector would have absolutely no power to have Margot temporarily released from Prison, especially prior to a death sentence. It would have taken several appeals and even if she was released to help with the investigation, she would have been handcuffed and accompanied constantly by police officers to prevent her escape, and definitely not allowed to wander freely under far-away surveillance.
From the position in which she was being forced flat on the desk while Swann was strangling her, Margot could not possibly have stabbed him in the back as far down, and at such a deep angle, as she is supposed to have done.