The man is incorrect when he tells the boy that they (the Nazis) will be bombing France with Big Bertha which has a range of 25 miles. Big Bertha was a nickname given to a gun the Germans constructed in the first World War. It was made by welding 3 gun barrels together which gave it a range of 75 miles.
The string quartet of Grace, Faith, Hope and Charity are clearly not actually playing the piece of classical music near the end of the film. Also the Mozart serenade in G K.525 (Eine kleine Nachtmusik) that they are playing is scored for five instruments, not four.