In all my patrols, I've only suffered one dud on a perfect 90ยบ shot. Unfortunately, it was a fuel bunker shot on a Bogue Carrier from 750m out with a T2.
What I seem to get a lot of, but still rare during a single career, are premature explosions during '39-'42. Nothing like having your first shot fired on a convoy explode prematurely and alert the entire convoy to your presence.
After '42, that can tip the small scale between life and death as you'd better start running and not try to be greedy.