At the end of Downton Abbey (2010), Carson had to retire as butler because he developed a palsy in his hands, and the shaking made it impossible for him to wait at table. In this film, Carson's palsy has magically disappeared and he is seen pouring wine for the family at the chateau with no problem.
They show two separate pieces of dialogue being recorded on the same disc. This was not possible at the time as no sound editing was possible.
When the sound film is being shot, the cameraman is shown hand-cranking the camera. This was not possible. All cameras were electric motors so the film and soundtrack synchronize.
Lucy Smith, on her wedding day to Tom Branson, wears a bridal veil. However, at her wedding breakfast, the veil is removed in favor of a gilt, crosshatch headpiece. This is factually incorrect and goes against the etiquette of the time period. Brides did not remove their veils at the wedding breakfast but kept them on until they removed all of their bridal wear in favor of the honeymoon/getaway ensemble. The crosshatch headpiece is more suitable for nightwear at parties, balls, or theatre.
The film appears to be set in Spring/Summer 1928. Jack points to a cinema showing The Terror (1928) and says it is the first full sound film, as The Jazz Singer (1927) only had sound in a few crucial scenes. The Terror was first released in America in September 1928 and Britain in October. It is widely acknowledged that Lights of New York (1928), released in July 1928 in America, is the first wholly sound film.
Isobel, on marrying Lord Merton, became Lady Merton. She would sign letters and be known by her friends as Isobel Merton. The style Isobel Grey, Lady Merton would be used only in formal legal documents and on her tombstone. The credit list is therefore correct.
As detailed in the trivia section, Robert and Carson look abnormally tan in the first scenes, before they go to France. This is because the scenes set in France were shot before the rest of the movie, and because of the pandemic quarantines, the actors had two weeks of free time, part of which was clearly spent in the sun.
The orchestrations for the songs played and sung at the house party are several decades too early.
When Lady and Lord Crawley, and the Marquis de Montmirail are boating, the name of the boat is using Zapfino font. That typeface was created by Hermann Zapf in 1998 (based on a 1944 alphabet), while the movie is set in 1928.
The film company's catering van has white 'blackout' markings painted on its wheel arches. These were painted onto vehicles to help reduce accidents during wartime air raid precautions. However the film is set in 1928 - 11 years before the outbreak of world war 2.
Countess Violet asks someone who is puzzled by what she has just said, "What bit of it don't you understand?" This wisecrack did not exist until the later part of the 20th century.
In the family photo taken at the wedding at the start of the film, Lucy is standing with her mother. Tom's only family is that of his late wife. The photo doesn't include his mother or brother (the two family members who have been discussed).
It is doubtful that Jack Barber would recognise Myrna Dalgliesh's accent as something that audiences would not respond to. Talkies were still fairly new, and no one knew yet what audiences would or would not accept.