
Continuity mistake: When Fink is explaining the difficulty of the boot on the chalk board, he draws one circle on the toe of the drawing, it then changes to two after he throws Bo Bo and says "Sorry" and changes back to one circle again. You can tell that they are two different drawings altogether.
Answer: Overconfidence. The Germans are convinced that they'll never master Das Boot, so they're too busy watching for the Americans to fail to concentrate on their own boot.
Captain Defenestrator