Factual error: In the scene towards the start of the film in Weymouth harbour, you can see the huge building which is Weymouth Pavilion, which was built in 1954, after the original 1909 building burnt down.
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: There are a number of scenes of the buildings along the coast where TV antennae are visible.
Factual error: In several scenes, but notably very near the end, the background shows numerous container handling cranes, which weren't invented until the mid sixties. (00:51:40)
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: Not until the very end of the 10-day evacuation were the French (or British for that matter) defending the very beaches themselves, and yet the movie opens with the French roadblock doing just that.
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: Velux windows were invented in German occupied Denmark in 1941. When burying a soldier on the beach, you can see them installed into French houses in 1940.
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: When Commander Boulton is looking out to sea at the civilian navy he has the binoculars backwards.
Suggested correction: No, he doesn't. The objective lenses are smaller than on modern binoculars but he definitely has the binoculars the right way round. You can tell he has the binoculars the right way round because the objective lenses are on the outside of the two barrels. The eyepiece is on the inside of the barrels.
To add to the above correction, you can see WW2 binoculars at https://globalwarmuseum.com/produkt/binoculars-british-army-1943-mk-iii-x6-taylor-hobson-perfect-optics/. Commander Boulton can be seen holding them correctly at https://fyeahkennethbranagh.tumblr.com/post/168344257844/dunkirk-2017-dir-christopher-nolan.
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.
Factual error: The Dunkirk beach lights are completely wrong - modern. In original photos they are black and of their time.
Suggested correction: While it is true the main German assault did not occur until near the end of the evacuation, what we see in the movie actually fits quite well with what really occurred. As the German Army approached, the French Army began to build defenses and roadblocks in and around Dunkirk, as well as within the city itself. The amount of gunfire we hear (just a few small arms shooting) at the beginning of the movie is indicative of a chance encounter between a German and a British patrol. While preparing for the final assault on Dunkirk, the Germans would be sending in scouting patrols to see what defenses were built and where they were located.
Firstly a scouting patrol would not open fire and give their presence away. Then the British patrol would not flee throwing away their weapons as they would know the city to be in Allied control with lots of Allied troops around. Barricades across the roads leading to the beach would obstruct Allied soldiers access to the beach, barricades would only be put up at the last moment and only to obstruct the Germans. The period is at least a week before the evacuation was completed, so the German army was miles away and there would be hundreds of thousands of Allied soldiers in and around Dunkirk. I am sorry but I had the distinct impression the Germans were only a short distance from the beach.
Having both studied and taught military history, within the military, for over 40 years, with particular emphasis on World War Two, in my expert opinion, what we see occur at the beginning of the movie is completely within the realm of factual history. Demoralized British troops have an unexpected encounter with advanced German troops entering Dunkirk ahead of the main attack to come; the Germans open fire with small arms; the demoralized British troops scatter in panic without resisting. Once it became apparent to the French they would be making a last stand defense at Dunkirk, the French did in fact begin preparing defenses. They would have been derelict not to have done so. While what we see in the skirmish at the beginning of the movie can't be pinned down to any actual engagement, it is plausible enough to be a factual representation of what occurred in and around Dunkirk at that time.
One of the things the film doesn't show is that in reality the town would have been heavily bomb damaged.
If you look very closely at the scene where Tommy's trying to load his rifle and fire in the back garden (or wherever it was) you can just about see a figure in one of the balconies in the houses in the street behind. So most likely a sniper. Also originally it was assumed the Germans were just 36 hours away when the evacuation started so it wouldn't be entirely out of the realms of possibility for a French street barricade to have been put up.