Factual error: When the ship is being launched a champagne bottle is smashed against the hull. Harland & Wolff, like many shipbuilders, did not smash champagne bottles against the hull.
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.