Errors in geography: In the shot when the small boats leave the boatyard, they are actually heading away from London and towards Teddington Weir.
Continuity: When the platoon come under air attack at the farm (and their officer is killed), the aircraft attacking them seen head on is a Blenheim, an RAF light bomber, where as the aircraft shown flying away is a German Junkers 88.
Revealing mistakes: The Vickers gun being fired by the Dunkirk rearguard is just having a belt of ammunition fed through it. Cartridges are still in the belt coming out of the gun (they would be ejected separately).