When the police first pull into Robbie's driveway, they park to the right side leaving the escape route to the left. Once Robbie's car has pulled out and the police give chase, they return to their car now parked on the opposite side of the driveway.
When Robie and Hughson are walking at the flower market, they look back at a few men following them. As they do, a woman in a bright pink dress is walking towards the two men behind them, but as they turn back, the same woman (who should have been behind them) is walking towards Robie and Hughson.
When the police first ask Robie to accompany them for questioning, he excuses himself to change into something more formal. He is seen going up to his room wearing white socks. He is later shown at Bertani's restaurant wearing the same shoes but no socks.
John is seen in a motor launch wearing a blue and white horizontal striped jumper, grey/blue slacks and a red kerchief with white spots. He swims ashore wearing tartan shorts and not carrying anything. He's next seen in a flower market wearing his original clothes.
The letter that H.H. Hughson gives to John Robie with the
names of jewelry owners is folded into thirds horizontally, but later at the beach, when John Robie notices it has a wet thumb-print, the letter is folded in half vertically. In addition, the Stevens' address block has changed from 3 lines to 4 lines. "Chambre 541" has moved from the 2nd line to a separate, new 3rd line.
During the last dance of John Robie and Francie at the party at the villa, the drummer of the orchestra plays with both hands in one shot, while in the next shot, he is holding up his right hand and only playing the drums with his left hand.
When Danielle is taking John to the beach club to avoid the police, they briefly stop the boat to discuss moving to South America. During the conversation, Danielle waves her finger at John, but in the next shot her arms are crossed.
On the list of jewelry owners, the room number of Mrs. Jessie Stevens is given as 541, but when John Robie accompanies Mrs. Stevens and her daughter to their rooms, the numbers on their doors are 625 and 623, respectively.
After Robie leaves Danielle's boat and swims to the beach, he lies down in the sand. Since his body is all wet a lot of sand sticks to it. Yet in the next shot when he is asked to answer the phone at the beach, there is no more sand on his body.
The note with the warning message on Carlton Hotel stationary that John Robie receives at the hotel's front desk before going to the Beach with Francie has the impression of a similar message on it, made when the prop department was creating various notes for the director to select for use in this scene.
When John and Frances first arrive for their "picnic" and John is standing at the back of the car, his shadow can be seen on the backdrop of the canvas, a flat shadow, which would not be possible in the natural conditions which the canvas depicts.
When our heroes watch the fireworks display from a hotel room, no light from the fireworks is reflected on any of the surfaces in the actual hotel room, despite the fact that it is a massive light display and they are watching it in the dark (because it's just a rear projection).
When Francie is retiring to her room after the party, it is obviously still nighttime, but the matte paintings (painted to look like the real outdoors) behind the villa windows in the hallway are representing daytime with blue skies and cypress trees clearly visible.
For the first 20 minutes or so, Cary Grant (as John Robie) is wearing a red cravat with white polka dots. At about the 18 minute mark Robie jumps in a boat driven by Danielle, to escape the police. In several angles on the boat, his cravat is solid red and then he goes below deck to avoid being seen by a helicopter. There is one shot below deck, probably a later re-shoot, where Robie is not wearing a cravat at all. But after putting ashore, he is wearing the red and white polka dot cravat again.