Factual error: When the returning soldiers are on the train at the end of the film, it's an open plan post-war British Rail Mark I type, which where built from around 1950. Also the blue upholstery on the seats looks to be the corporate blue introduced by British Rail in the 1960s, used by the preserved railway owning the stock, and not what would have featured in Southern Railway carriages of the time. The carriages also have horizontally-sliding windows, which are far more contemporary than wartime trains, which had windows with a much larger vertical opening, held in place by a leather strap.
Factual error: When the boat returns to England they state the cliffs are Dorset. There were 3 evacuation routes from Dunkirk - all to Kent. It would be nonsense to sail from Dunkirk to Dorset as you have to almost pass Dover on the way. Never mind the fact the boat would probably not have made it without refuelling.
Factual error: In the silent overfly of the plane that ran out of fuel in the background you see a lot of modern architecture that is definitely not from the 1940s. (01:30:15)
Factual error: Near the start of the film in the harbour when the camera pans around to the left looking out to sea, you can see the back of the modern Weymouth Lifeboat 17-32 moored up in the background.
Factual error: The Spitfire had enough browning MG ammunition for 16 seconds, hence pilots shot in very short bursts. In the film at least 30 seconds of fire came from one aircraft.
Factual error: When told instructed by Fortis leader to stay at 500 feet to leave 40 mins of fighting time. Pilot confirms saying ".5" however for altitudes under 1000 feet "cherub" would have been used - "cherub 5" being 500 feet.
Factual error: One of the dams in or around Dunkirk has a foundation made of tetrapods. These concrete structures were developed in the 50s, according to wikipedia (https://en.wikipedia.org/wiki/Tetrapod_ (structure)).
Factual error: Oriented strand board (OSB), also known as sterling board, sterling OSB, aspenite, and smartply in British English, was not invented until 1963. The French warehouse floor was covered with it. See blood spot close up.
Factual error: Colonel Winnant wears a regimental cap badge. In fact, full colonels (as he is) and brigadiers have a different cap badge (a lion standing on a crown). It's not entirely beyond the realms of possibility that he would choose to continue to wear his old cap badge, but it would be very unusual (and completely against regulations).
Factual error: The Heinkel He 111 defensive rear gun sounds are ridiculously low and slow (pom pom pom). In reality fire rate of the 7.92mm and 13mm guns was much faster.