When Valiant flies down the gun barrel and into the bunker, he drops his canister at the breech, yet when he leaves out the same gun, his canister is nowhere to be seen.
When Wing Commander Gutsy and his flight take off from the pub, his "jet wash?" (for lack of a better term) pushes Valiant and the rest of the patrons backwards yet the recruiting fliers he leaves behind are all blown forwards over the bar.
The German falcons wear "Pickelhauben" (pointed helmets) which were worn during WWI not WWII.
In the movie members of the French Resistance, play "Non, je ne regrette rien" sung by Édith Piaf. However World War II was over twelve years before the song was even written.
The steam locomotive seen towards the end of the film whilst Valiant is being chased by Talon is painted in an odd mix of 1950's British Railways Express Blue (Loco) and pre-war LNER Apple Green (Tender). With the film being set in 1944, the locomotive should be entirely painted black as per wartime measures set out by the railways.
Valiant calls his instructor "Sir". In fact the instructor has the Rank of Flight Sergeant. You wouldn't call an NCO "Sir". You address him by his Rank (in this case Flight or Flight Sergeant).