Continuity mistake: When James Coburn is about to leave the phone booth where he's been lighting matches, the number of matches he leaves on the top of the phone changes between shots.
Continuity mistake: Charlie's fingers, when he is dead, change positions when suspects arrive for his funeral. (00:17:00 - 00:19:00)
Continuity mistake: When the French inspector is interrogating Audrey Hepburn, he asks "Why did he want to leave France?". The shot changes to the back of the inspector and Audrey replies, "Leave? No." Then the inspector makes a rough movement with his body and head to his right, but the movement is brusquely cut and replaced by a front shot where he is sitting perfectly still, as he was doing in similar previous front shots.
Visible crew/equipment: As Reggie and Sylvie are walking around the indoor swimming pool, the boom arm and mic are reflected in the glass as they turn the corner.
Factual error: The sign outside the embassy reads "American Embassy", it should read "Embassy of the United States of America". North & South America are continents, not a country.
Character mistake: When Audrey Hepburn confronts Carrie Grant by saying "Carson Dyle had no brother.", he would have definitely wondered how she came to know about this, and who is her hidden source of information. (01:16:00)
Continuity mistake: Towards the end of the film, 'Adam' meets Reggie at the meeting where she is working as a translator. They hurriedly leave for the venue (gardens) where her late husband had his last appointment. She leaves her friend, Silvie (who has several appearances throughout the film) also working there as a translator. Yet no sooner have they arrived at the gardens than Reggie sees Silvie sitting on a bench reading a magazine, waiting for her son to return from the stamp market. How could she possibly have got there so quickly?
Plot hole: Reggie finds Tex's body stretched out on the bedroom floor, his feet tied to the radiator next to the wall and his hands bound to the bed frame, suffocated with a plastic bag over his head. The bed is a twin-sized, wood frame that Tex, a tall, strong young man, could have pulled the end of it with his hands and arms close enough to his face to at least attempt to tear a hole in the plastic bag so he could breath. He was obviously conscious early on because he wrote the name "Dyle" in the dusty floor with his finger. Considering the floor is covered in dust, there's not even the slightest sign of a struggle to free himself. Instead he just neatly prints the name on the floor before he dies.
Continuity mistake: In the bedroom, Cary Grant puts on his glasses to read a note. When Audrey Hepburn says "I love you, Adam", he turns around and doesn't have his glasses on anymore. Next shot, they're in his hands. (01:25:20 - 01:26:00)
Continuity mistake: While Cary Grant and Audrey Hepburn are on a boat going down the Seine, they have two drinks in front of them on the table. The problem is: the drinks keep changing, from full, to two-thirds full, then back to full again, both of them, over and over.
Continuity mistake: When Cary Grant and George Kennedy are fighting on top of a building, Grant is hanging over the edge, but the rooftop sometimes has a metal framework on it and sometimes it doesn't.
Plot hole: When Walter Matthau is interviewing Audrey Hepburn in his "office," she is looking at a picture. She identifies various faces in the picture. Matthau correctly identifies the person without knowing which face she was asking about. Other than her husband, he could not know which of the other three faces she was studying.
Suggested correction: I believe Matthau/Dyle handed the pictures to Hepburn and knew the order in which he had put them, and therefore who was in each pic: first, second third.., as she went through them in order from the top.
Plot hole: When Cary Grant is chasing after Audrey Hepburn where she has to meet Walter Matthau outside the Paris Opera he shouts "Reggie stop - that man is Carson Dyle". Tex recognised him, that's why he wrote Dyle on the carpet. But Cary Grant hadn't gone back into the room at the hotel, so he couldn't even have known Tex was dead.
Suggested correction: While Cary Grant doesn't see 'Dyle' written on the floor, Cary Grant sees Audrey Hepburn after she's exited the room and while fleeing she yells out that 'Tex is dead and wrote Dyle on the floor and you're the murderer' (paraphrased). That's how Cary Grant can refer to it in the next scene.
Suggested correction: You must have seen an edited version of the film. Not only does Grant go back to the hotel, but Hepburn explicitly tells him that Tex is dead and he wrote Dyle on the carpet.
No, Hepburn believes Grant is the killer - she phones Matthau (who she still thinks is the embassy guy) to tell him what Tex wrote on the carpet. When she leaves the room, Grant is coming up the stairs as she descends in the lift and he chases after her. She wouldn't have needed to try to get away from him if she'd already told him this.
Check out of the specific dialog in the scene: "Reggie! The stamps! Where are they? Reggie! Wait!" "Why? So you can kill me, too? Tex is dead. He wrote Dyle on the carpet." "I'm not Dyle. You know that." "But Tex didn't know it. You're a murderer." If your version of the film is missing this exchange, you have an edited copy. Since the film is in public domain, that is possible.