Factual error: The BEF was a professional army and was not staffed with conscripted prisoners.

Deliberate "mistake": In the approach to Dunkirk, Robbie is carrying a pistol, complete with webbing holster. He has no rifle and is carrying a bayonet on his haversack, presumably not having the usual belt frog to carry it more easily. As a Private (this is emphasized in the dialogue) in an infantry regiment, he'd have carried a rifle and not a pistol. Although he might have acquired the pistol during the retreat, he's less likely to have changed his webbing order to match. This may be a deliberate directorial choice. The dialogue emphasizes his natural "officer qualities", and that his prison record has prevented him from becoming one. The wearing of officer's equipment, even if historically inappropriate, also reinforces this and distinguishes him from his comrades.

Continuity: In the love making scene in the library, Robbie pulls down the strap of Cecilia's dress and she pulls her arm out. In the next shot it is up and over her shoulder again.