Anna gets ink on her white gloves whilst signing an autograph in front of the Player's Club. Yet in every scene after that, her gloves are spotless.
In the breakfast scene, the purple-flowered potted plant on the windowsill changes from having tall, spindly flowers to much shorter flower stems and then back again.
The balls on the Snooker tableware in same position when Grant and partner leave the room as it was when they started playing.
When Anna and Philip arrive back at Anna's apartment after their first date, Anna lights a fire in the fireplace. First the flames are abundant and high, yet moments later as they talk and walk around the living room, the flames are barely noticeable. However, as the camera pulls out showing Anna alone in her living room after Philip has left, the high flames have returned, and the fire is roaring once again.
Anna puts her gloves on the back of the chair, and Philip puts his coat over the gloves. However, when he pulls his coat off the chair just before leaving, the gloves have disappeared.
About 24 minutes into the movie, the Big Ben clock tower shows six o'clock, and a character remarks that it is six o'clock while its chimes are heard. However, seven clock chimes are sounded, not six.
When Philip leaves Anna's apartment, one gets a straight on glimpse of the living room from the entry door. Straight ahead is the fireplace. This can't be possible because in all other scenes, the windows and balcony are straight ahead, and the fireplace is to the left of the entrance.
When the two sisters are talking in the back of the Rolls Royce, the back projection shows them leaving Euston Station. About 30 seconds later and with no break in their dialogue, they are shown driving round Trafalger Square, a distance of 1-1/2 miles that would take a minimum of five minutes, probably nearer 10 - 12 minutes in those days.
A boom microphone shadow is visible when Anna cooks breakfast.
Cary Grant's character states that he is from San Francisco, with the implication that he is a native San Franciscan, but his accent makes that exceedingly unlikely. Similarly, Ms Bergman's character has a distinctly non-British accent, while her "sister", played by British actress Phyllis Calvert, has a typical plummy RP accent.
When Anna telephones the front desk to stop Philip, she asks if he's gotten off the elevator. In England elevators are called lifts, and as she says London is her adopted home, she should know that,
When Anna and Philip are having their first dinner, Anna states she watched "Camille" and wanted to do a stage play of the movie at her school, Anna says "....with me playing Camile of course", the main female characters name in "Camile" isn't "Camile" but "Margarette". Camille is just the title of the movie and the flower that Margarette carries alongside her throughout the movie.