Continuity mistake: After Kurt returns to the quay from the raid near the start, the coat of one of his men is noticeably bloodstained. Later, walking up the stairs to the pub, the bloodstain is gone. But when he pours alcohol over Devlin's arm, the bloodstain has returned.
Factual error: When Michael Caine is standing outside the village church, it is clearly visible that double yellow line road markings have been blacked out (covered up). The film is set during the war, but double yellow lines did not make an appearance in the UK until 1958, therefore it is clear that the filmakers blacked them out to avoid an anachronism.
Continuity mistake: After Donald Sutherland parachutes into England, he walks across a cow paddock towards a gate. As he climbs over the gate, you can see there are no cows to the left of him along the fence but in the very next shot, you can see a line of cows along the fence which weren't there in the previous shot. (00:39:55)