When Foyle and Milner sit down to dinner with the women there is a dish of Brussels sprouts on the table. These are a winter vegetable who's season is over by March. Elsewhere we are shown fields of bluebells. These flowers normally appear late April to Mid May.
The lavatories in the POW camp had chain pulls which would have been replaced with straight rods to prevent them being used as weapons.
When the german plane crashes, the crew reports over the radio "64 degrees north, 4 degrees south" in German. A location cannot be both north and south at the same time, but is told in degrees latitude and longitude.
The photo portrait of King George VI, famously taken by Yousuf Karsh, is seen hanging on the wall of the village shop in April 1941, but wasn't actually taken by Karsh until 1943. (The same photo is also seen hanging in Foyle's office in many episodes.)
Schimmel introduces himself as an Oberleutnant, but is wearing the junior rank of Leutnant (one wing over a bar). Weiser wears the Oberleutnant rank (two wings over a bar), as seen when hanging in the tree, and is referred to as Leutnant. Perhaps the actors swapped flight uniforms for a better fit.