I was on the 2nd patrol of a new career & I think that I experienced the closest "shave" that I've ever had playing SH III. It was July '41 & we had intercepted a large convoy in BF 14. I got us inside the convoy & past the first escort without being noticed. I lined us up on an Empire-Type Freighter and launched a T1. That one hit her and I was all set to deliver a second shot when I heard a big metallic sounding "CLUNK". I had a look around & it turned out that we had bumped into a Flower Class corvette!
At that point I thought that's it, we're dead. But apparently she didn't even flinch; just passed by without dropping anything at all. I never did get a chance to finish the freighter off as the collision screwed up my aim & I didn't want to push our luck so we went to silent running and headed for the deep. I figure that we must have passed through the asdic's blind spot (I remember reading somewhere that it was an area around 500 feet from the asdic equipped vessel).
Lesson to new players (and maybe some old ones too): don't allow yourself to get fixated on your target.