In the scene where the big Nazi Destroyer sends a boat to the sub, they blow up the radar with a big gun. When they set it up they take a cap off of it. After firing and going back below deck, they forget to put this cap on. In the very next scene where the are squeezing under the ship, the cap is back on.
It's a slip up. Out in hostile waters, in a German U boat, about to shoot a BB gun at a tank. Did you notice how frantic they were? They didn't intend to put the cap back on after firing. What would you have done? Worried about the gun or your happy little ass?