Revealing mistake: When McKay and Ramon are riding into Blanco Canyon to rescue Julie, you can see a diesel loading shovel with its bucket raised in the bottom of the frame of one of the scenic shots. Presumably the scene was shot in a quarry and they didn't get all the equipment out.
Revealing mistake: In the final scene in the belltower you can see shadows from the surrounding trees. That's because this scene was shot at 3 in the afternoon. They just used a filter to make it look like it was night.
Revealing mistake: When Corporal Bin's group of men eventually arrive at Dunkirk, they speak to a soldier adding sandbags to a Vicker's gun position. Here (and later on in the film when it is shown firing) it has a blank-firing adaptor prominently fitted to the front. Additionally the belt of ammunition shown going through it is filled with blanks - no bullets project outside the other side to that of the actual cartridge cases.
Revealing mistake: Whenever our heroes are shown firing the Vickers guns on or off the vehicles, they are very obviously fitted with blank-fire adapters to the front. The belts of ammunition for them throughout the film are also blanks - the cases are visible, but the bullets that should project out the other side of the belt are not present.
Revealing mistake: When a yellow cab pulls up to the curb, it is obvious that the street is part of a studio backlot and not a real city street because the businesses have no names. The sign outside one building says simply "Tobacco." The diner has only the word "Restaurant" painted on its windows. Another business has no identification other than "Fish and Poultry" painted on its window.