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.