Factual error: Two lumbering, obsolete German trainers (Messerschmitt 108s, made in 1938) attack the RAF airfield, shooting it up unopposed. By early 1944 (the time this film was set) the Luftwaffe was a tattered remnant of its former self and the RAF had complete control of the skies over the UK and most of France. Any German pilot foolhardy enough to try such an attack would be shot to pieces before he got to the English Channel, let alone Southern England.
Factual error: In all the scenes in which the '109s' attack, you can see that they are not Messerschmitt 109s but the Bf108, a civilian/training aircraft (the 109's predecessor).
Factual error: At the beginning of the film we see a damaged Mosquito being examined by the pilots and other squadron members. The holes in the fuselage and tail show the bent edges of damaged metal, but those surfaces on the Mosquito were fabric covered. (They made a prop metal plate to fit over a real Mosquito).
Factual error: Near the beginning, after the initial return of the squadron, when Wing Cmdr. Roy Grant is taken to his superior, as the jeep drives away, between the two hangars in the background can be seen a light blue/grey Series 2 long wheelbase Land Rover, from the late 50s, early 60s.
Factual error: At the very beginning of the film three Mosquitoes 'buzz' the airfield, pulling some fancy aerobatics while in close formation. Had that happened in real life in World War 2 the pilots would have found themselves under close arrest the second their feet touched the ground. Such displays were reserved for airshows - not that they had them in those days - due to the risk of collision. Aircraft at the end of their missions were required to fly straight in and land without fuss and without showing off like barnstormers, which wasted fuel and the time of traffic controllers and other ground crew.
Factual error: The dialogue refers to "Earthquake bombs" The real earthquake bomb, Barnes Wallace's Grand Slam weighed 10 tons and was designed to penetrate deep into the ground, shaking structures above. Even that would be unlikely to have an effect in the posed scenario.