Perhaps the well knows Mk 14 depth issues? Early war Mk14s have difficulty hitting destroyers in calm seas: either with default or minimal depth setting they will, more likely than not, go under the keel without detonating.
I clearly remember setting a Mk 10 to shallow as it go then while watching it with the external cam noticed it ran literally just below the surface. I think it could even contact-detonate on Gun Boats if it runs like that all the time.
EDIT: Here's a shot taken from my latest convoy encounter. Contrary to what you guys might think, there are just two ships in the picture: a Gato class and an ex-Destroyer.
