In the plaza scene when Lucy faints, her photographs do not fall into the blood, her hat does. The hat rolls to the left into the blood pool. When George retrieves the packet, the hat and the packet have switched places.
In Mrs. Vyse's home, she and Cecil Vyse discuss Lucy becoming part of the family. The mantel clock reads about 10:46. When Cecil and Lucy part on the landing, the clock strikes 10.
Freddy Honeychurch's hairstyle as he plays the piano.
At the picnic in the Florentine countryside, the light and shadows on the faces of Charlotte Bartlett and Eleanor Lavish during their conversation change between shots.
The hand that Charlotte Bartlett holds her purse in when she falls over the bicycle at the station.
In the plaza scene when the man who was killed in the scuffle falls to the pavement, a cigarette butt with a filter is shown between the bricks. Filters were invented in the 1920s and were not in widespread use until the early 1950s.
The very highlight of the soundtrack is Puccini's beautiful aria "O mio babbino caro", from the opera Gianni Schicci - even though the story is set in the Edwardian era (very early years of 20th century) and the opera in question wasn't premiered until 1918.
As the steam train pulls into the station, in the background can be seen a wagon/car in British Rail blue with yellow roof, and also a self-propelled diesel rail-car.
The psalm chant sung by the choir was written after the purported date of the film.
Lucy uses a two-handed backhand while playing tennis. This stroke wasn't introduced until 1930, while the movie is set around the turn of the century.
In the opening, Lucy and Charlotte complain that their rooms face north, and thus they have no view of the Arno river. They switch to south-facing rooms. But the view out of their new window faces both the Arno and the Duomo, the latter of which is north of the Arno. Thus, the new rooms must face north; it is the only vantage point from which one may view both the Arno and the Duomo.
When George throws Lucy's bloodstained postcards into the Arno and they float over a weir in the river, the backdrop is the Ufizzi Gallery which is nowhere near either of the weirs along the river in Florence.