After Stefan has climbed into his single-engined plane for the flight to Romania the scene merges into a take-off shot, but the aircraft taking off is twin-engined.
The in-flight close up of a German bomber over England is a reversed shot as indicated by the plane's registration letters.
In a cockpit close-up, during the final air battle, the "Poland" flash on Stefan's uniform is back-to-front, as in a mirror image. This indicates that the original left-to-right shot was flipped so that it coincided with the other right-to-left shots that are used in the complete battle sequence.