Revealing mistake: In some of the shots where multiple British soldiers are together, most have their rifles (Short Magazine Lee Enfield, No. 1 Mk 3) slung or are holding them. Some of the rifles are dummy, toy rifles that look nothing like the Enfield and are just to stand in.
Deliberate mistake: Christopher Nolan admits he used a French destroyer instead of a British destroyer, for practical purposes (most people won't notice). The giveaway is that it has a closed bridge. British warships were built with open bridges till after the end of the Second World War, as can be seen by looking at HMS Cavalier (http://thedockyard.co.uk/explore/three-historic-warships/hms-cavalier/), which was built in 1944.
Character mistake: The story arc set on the Mole covers one week. The Highlanders attempt to refloat the beached trawler on the last day of that week. One of them confidently states that the tides are three-hourly. (The audience know this isn't true because we've seen a similar conversation between the Admiral and the Colonel.) At least some - if not all - of those Tommies have been on the beach all week. Have none of them have noticed that the tides are six-hourly?
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.
Other mistake: In a scene where the commanding marine officer stands on the mole in a close-up shot, black smoke is rising from sky out of nowhere. Looks like the FX guys forgot to delete the particle effect after deciding to not show a boat with its exhausts in the background.
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.
Continuity mistake: As the lone Spitfire trails and attacks the German bomber, he hits the right engine, which starts smoking but the trail quickly fades. He then hits the left, which smokes a lot. We then see the same attack from below, and from this angle the both engines are smoking equally badly.
Continuity mistake: When the two British soldiers are exchanging a bottle of wine, it's full in one shot but when given to the other soldier it's suddenly less so.
Continuity mistake: During the spitfire scenes over the channel, it's day light, however, when one of the ships was bombed and the soldiers jump over board, it's pitch black. Several cuts between these scenes each time show daylight in one and night in the other.
