On the beach waiting for the German plane, the two lines of schoolgirls are nowhere near far enough apart for the plane to land between them, but their flare-lit lines appear from the approaching plane's cockpit to be the proper width.
When Judi Dench goes into the classroom and sees "Jew" and "Gentile" written on the board, the writing is different from when Ilse first wrote it.
The closed caption text for the radio broadcast announcing that Great Britain is at war identifies the speaker as Winston Churchill. It was Neville Chamberlain.
The phone box Miller used was too remote from the nearest exchange to work without overhead wires, underground cabling would not be used to service it in 1939.
The scene of a coastal excursion on the bus has figures paddling on surfboards in the distance.
The Bedford Ob/Duple bus driven by Charlie was not manufactured until 1949.
By September 1939, the UK radar early-warning system (Chain Home) was fully operational and had been effective for some years. The area around Bexhill, being particularly well covered and able to detect aircraft down to 50ft above the water. It is unthinkable that an aircraft of the size of a Ju-52 could get anywhere close to landfall without being intercepted.
The RAF Spitfire sent to intercept the Junkers Ju-52 transport aircraft has the blue-grey underwing paint scheme adopted in Sept 1940, a year after the events of this film. It should have the half-black/half-grey scheme adopted from 1938 to help avoid friendly fire from anti-aircraft defences.
The Junkers Ju-52 transport aircraft has markings introduced in 1940 after the Battle for Poland. This is set months earlier.
The Ju-52 transport aircraft is intercepted by a Spitfire, at night. Spitfires were not used as night fighters in 1939, only briefly during the summer of 1940, a year after this film is set.
Although the film is set in Bexhill-on-Sea in East Sussex, the pier used is in Penarth in South Glamorgan on the south coast of Wales. It is around 215 miles from Bexhill.