Before the party, when Meg polishes her nails and later she puts on her gloves. Afterwards she puts on her earrings and shoes, the gloves are gone.
On Darlington's yacht near the end of the film, Mrs Windermere's wrap moves back and forth across her shoulders in two-shots with Mrs Erlynne.
When Meg finds the check stubs for checks written to Mrs. Erlynne, all of the check stubs have the same check number on them. They should be sequential.
Cast members are shown smoking filter tipped cigarettes which were not available in the 1930s.
The band playing at Mrs Windemere's party includes a guitarist playing a Gibson electric guitar (Gibson pearl-inlaid logo is visible). This is 6 years before Gibson released its first commercial electric guitar (the ES-150) in 1936. Electric guitars were an experimental novelty in 1930.
The instrument being played in the band at the party is not a cornet, but a flugelhorn. In some of the musical selections, the sound heard is that of an unmuted trumpet, in others, it is that of a trumpet with a Harmon mute. The flugelhorn, as shown has an entirely different timbre.
In the scene where Meg is painting her nails (after discovering her husband has been writing checks to Mrs. Erlynne), the nail varnish clearly has a gray plastic lid, which wouldn't have been available in this era.
The aircraft in the last scene was a De Havilland Dragon Rapide DH-89A, noted by its nose landing light and the wind powered generator on the right wing leading edge. These aircraft were built sometime after April 1934. When Mrs. Erlynne handed back the check, the date written was September 1930.
When the band is playing at the party, the trumpet (actually a cornet) is shown without a mute, but the soundtrack is of a muted trumpet. The tone heard is not possible otherwise.
In a brief shot before an evening party, what is presumably intended as a sunset is shown, with the coast on the left and the water on the right. In Amalfi this is only possible facing East.