When the schoolgirls are shooting their arrows in the archery class (c. 61 minutes) they are fired at an angle of about 45 degrees. Because of the effect of gravity these arrows would fall well below their targets and not, as in the film, hit them with so much success.
When the PT boat is rescuing survivors and they spot a U-boat, the young boy who was being hauled aboard the PT boat is wearing a life-jacket. The close up in the next shot shows Lt Stewart lifting the boy up to put him back in the lifeboat. The boy is no longer wearing the life-jacket. In the following shot, as Lt. Stewart completes putting him back into the lifeboat, the boy is again wearing the life-jacket.
As Lt. Ward Stewart arrives on the dock to meet his new command, two Gato class submarines are seen leaving the dock. From the camera on the dock, they are traveling at, roughly, full reverse (stern first). The shot then alternates several times to the boats obviously moving at, more or less, full ahead (bow first). It's pretty easy to tell which end is which on a WW2 era submarine.
When the raiding party lands, the German guards are patrolling with bayonets fixed (which they shouldn't be doing anyway). The next time you see one of the guards moments later, the bayonet has disappeared.
When the submarine submerges after being ambushed by the Q-boat, they are lying on the bottom and the captain passes the word that 'the smoking lamp is lit' for ten minutes. Since WW-II era submarines did not have any form of air purification equipment on board, no submarine commander would ever authorize smoking on board whilst underwater, as this would quickly use up the limited air supply on board the boat. Smoking was only allowed while surfaced and the men had to be outside the hull, either on deck or a specific section of the after section of the conning tower known colloquially as the 'cigarette deck'.