Homebound to France from Canadian waters, Peter Cremer in U-333 came upon a zigzagging freighter on January 31 1942. Cremer submerged and approached to 400 yards to look the ship over. Cremer concluded that the ship was British. Cremer fired his last to torpedos at the freighter. After the first torpedo hit,  the ship sent out an SOS in plain language, and identified herself as the Brittany. but the ship was actually the German blockade runner Spreewald, homebound from the far east with a cargo of rubber, tin and 86 British prisoners she had received from the German merchant raider Kormoran.

       From the open radio broadcasts Donitz and the OKM divined the error immediatly, OKM was furious. Given Germanys need for raw materials Donitz considered the sinking of the Spreewald as an "Unforgivable error" Donitz directed that when Cremer returned to base he was to be immediatly court-martialed. The charges included "disobedience in action, manslaughter, and damage to military property.

       When Cremer arrived in Lorient on Feb, 9th, he faced an instant court-martial. But after all the facts had been gone over, Donitz's first staff officer, Gunter Hessler, (Hessler himself was a U-boat commander and winner of the Knights Cross, he was also Donitz's son-in-law.") rose to Cremers defense. The sinking was indeed regrettable, but the simple fact was the  Spreewald was not where she was supposed to be. Therefore Cremer was not at fault. That Cremer had conducted an outstanding first patrol and showed great promise weighed heavily in his favor. The court acquitted him; and everyone involved was sworn to secrecy, the affair was hushed up for years..

        It was Cremer himself who brought the whole incident to light in his book "U-boat commander" in 1982.

      I got this information from the book; "Hitler's U-boat Wars." by Clay Blair