Just after Mrs. Miniver hands the German pilot a bottle of milk to drink, spilled milk appears all over his coat. The milk subsequently disappears and reappears on the coat several times between shots.
In the radio broadcast of Lord Haw Haw, he mentions the fall of France. Then a day or so later, the boats are called out for the Dunkirk Rescue mission (Operation Dynamo). France did not fall until 2 weeks after Dunkirk.
The "double decker" bus seen in the opening sequence is not a British bus at all, nor was it actually a double decked bus. An American bus was used, with a false upper deck grafted on to it. The American-style passenger door can be seen on the right-hand side in the bus's first appearance; a real London Transport bus would have had its door on the left-hand side.
During the scene in the bedroom when Clem arrive back from Dunkirk they mentioned a meal called ham and eggs the British do not eat ham and eggs it's bacon and eggs she also little cigarette from the book of matches.