I will admit that I sank one out of pure frustration once. It was a convoy attack gone bad, then to worse. It turned out to be a primarily neutral convoy, with Vichy French, Portuguese and Swedish ships, with a French escort and one British hospital ship.
It was so dark all I could see was darkened ships and one brightly lit British hospital ship, and with the escort, I assumed it was a British convoy.
I had to get closer and closer to make anything out, until I saw so close that I saw they were all flying neutral colours. I was then angry for having wasted my time and fuel on the pointless hunt. Without realising I stupidly got to close and had a near miss with a French Merchant that slight damaged my hull and at this point I was throwing my keyboard and mouse around cursing the French!

So I sank the Hospital ship in a rage, only to feel terrible and I had to reload with the loss of half my patrol.....
Not my brightest moment..