(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.
When Captain Lesgate/Swann arrives and Tony pours two glasses of brandy, they both go and sit down and Tony does not bring the bottle with him. But later, as they are continuing their conversation, Swann refills his glass from a bottle and puts it down on the table.
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.
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.
When the silk stocking is burning in the fireplace (after Wendice puts lighter fluid on it),
he supposedly puts more on it. The fire doesn't change nor does it flash back, indicating
that there was no fluid.
The "scissors" that become the murder weapon are actually dressmaker's shears. They appear to be about a foot long. It is highly unlikely these would be in Grace Kelly's mending basket. One would use very small scissors for mending, and it's hardly likely that her wealthy character, dressed in elegant and clearly expensive outfits, makes her own clothes.
During the montage of the trial, Margot shakes her head no to the various incriminating questions; in a real trial she would have been required to answer verbally.
Some viewers report that (at around 11 mins) in the film, Margot calls Mark (Robert Cummings) "Bob". Others insist that she is saying "Mark" in a muffled whisper that only sounds like "Bob".
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.
Margot's press cuttings are clearly blank on one side; cut out of newspapers, they should have printing on both sides.
When Margot shows Mark the second blackmail letter, she starts to say her next line before her cue.
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.
When Inspector Hubbard is asking Margot about her latch key from her stolen handbag, Wendice starts to say his line and is cut off by Hubbard. He then restarts his line after the Inspector finishes speaking.
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. Close inspection of faces suggests that a double was used for Grace Kelly in the rear projection. When Margot enters the apartment from the street, the background is an obvious still as the two policemen and pedestrians are motionless.
Chief Inspector Hubbard uses small flashlight to show light on to the door lock in the dark room when Margot Wendice is trying to open the lock. The actual light source is coming from the direction of the camera, the movement of the light and the movement of the Chief Inspector Hubbard's hand is different and the light is circular, if the light were shown from the flashlight it would be ellipse.
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.
In a shot of the Queen Mary, a part of the wall holding the backdrop is visible.
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).
When Tony phones the police 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." (Of course, it could have been an American operator who happened to be working in London.)
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 trial, Chief Inspector Hubbard visits Tony to execute a complicated ruse that he had prearranged with Sergeant O'Brian (at the station) and Detective Williams (stationed upstairs). The ruse relies on the inspector mentioning Margot's handbag and switching raincoats with Tony. The inspector would have left the apartment without doing either of those things had it not been for Mark being present and calling out to the inspector, but the inspector would not have expected Mark to be in the apartment.
For the homicide charge on Margot, the investigators never consider how a shorter Margot was able to overpower a 6' 2" tall, well-built man, Swann, to stab him in the back. The peculiar hand motion directed towards the back of his feet, could not have been achieved by simply sneaking up on him behind his back and stabbing him.
[Around 0:44:15] During the scuffle in which Margot managed to stab her attacker, the scissors were initially stabbed with a hand motion in the direction of the back of his feet. Thereafter, he falls to the floor, which drives in the weapon further, which eventually kills him. The peculiar angle of the stab wound- facing the back of his feet, and its deviation at the time of impact on the floor, thus, would have been corroborated by the forensic investigators, which would have validated Margot's statement of killing Swann in defence. Hence, there would have been no murder trial on Margot in the first place.
Chief Inspector Hubbard says he couldn't get into the apartment using the key in Margot's handbag so he never saw the checkbook. Then he says it took him half an hour to find the correct key under the stair carpet. Thus, he eventually got the key, so why didn't he look at the checkbook then?
The key-exchanges, both deliberate and unintentional, would never have happened had they (the keys) been kept like it's generally custom to keep them - attached to a key-chain or something identifying your key(s).
The boom mic is reflected in garden doors above Tony's and Inspector Hubbard's heads during the Inspector's first visit.
During the trial the date of the murder is clearly stated as September 26th, but when looking at Tony's checkbook, Halliday says that March 26th was "the day before all this happened" and that Tony has been living on the cash in the briefcase "since the 27th of March".
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.
[Around 105 minutes] When Inspector Hubbard reaches to open the door after Mark presses the buzzer, the Inspector's hand slips and he briefly has trouble opening the door.
Chief Inspector Hubbard forgets to show either Tony or Margo his ID when he visits them at their flat for the first time.