Several takes are wasted trying to get a cat to drink milk from a tray. Eventually Joelle brings in "the studio cat" to do the scene. But the cat that drinks the milk is actually a third, different cat.
When Ferrand is talking to Julie in her room, his left ear appears without a hearing aid for a second.
When the director and the producer are browsing promotional photographs of the English actress, the actual name of Jacqueline Bisset can be seen briefly at the bottom of one of the photographs.
In the swimming pool, the shadow of the real crew and camera crane falls several times on parts of the set where action is going on in the film-within-the-film.
During the filming of the car crash, when the stunt man backs up the car driving through the the open door, and when he jumps free as the "driverless" car supposedly heads for the cliff where it will crash, someone else is visible inside the car, actually driving.
Early in the film, when Ferrand (François Truffaut) uses a telephone, he correctly holds the handset's earpiece against the microphone in his hearing aid, while keeping the mouthpiece near his mouth. When he later uses a telephone, however, he holds the mouthpiece to his hearing aid and the earpiece to his lips.