I May Have Made a Mistake.
 

So I was playing Silent Hunter V with the Open Horizons II mod. I was just north of the area of operations in which I had to sink shipping, and I noticed a contact that I couldn't identify. Upon closer inspection, I saw that it was a submarine. I got a bit flustered and immediately tried to put its information into the TDC, but the XO wouldn't allow me to identify it as a specific class of ship; it just called it an "unknown ship." I was fairly sure it had seen me, so I surfaced the boat and took some shots at it with the deck gun; I hit it twice with AP shells and a few times with HE shells before it registered as a sunk ship. Then I realized several things:
-It had almost certainly seen me first, but not fired.
-When I began firing upon it, it did not dive or fire back.
-It was classified as a U Type submarine.
...Did I just sink a friendly sub?

Not a friendly. A British sub.

Yes, at first it can be disturbing.
But looking more closely with UZO, it didn't have the look of a German VII...