This sub sank because its commander couldn't flush his toilet
In April 1945, being a German submariner was a dangerous prospect. Allied sub hunters had become much more effective and German u-boats were being sunk faster than they could be built. Technical breakthroughs like radar and new weapons like the homing torpedo were sinking the Germans left and right.
For the crew of U-1206, the greatest threat was actually lurking in their commander's bowels. German Navy Capt. Karl-Adolf Schlitt was on his first patrol as a commander when he felt the call of nature and headed to the vessel's state-of-the-art toilet.
While Allied subs had toilets that flushed into a small internal tank that took up needed space in the submarine, the Germans had developed a compact system that expelled waste into the sea. The high-tech system even worked while the sub was deep underwater.
Unfortunately, the toilet was very complex. By doctrine, there was a toilet-flushing specialist on every sub that operated the necessary valves. The captain, either too prideful or too impatient to search out the specialist, attempted to flush it himself. When it didn't properly flush, he finally called the specialist.
The specialist attempted to rectify the situation, but opened the exterior valve while the interior valve was still open. The ocean quickly began flooding in, covering the floor in a layer of sewage and seawater. The specialist got the valves closed, but it was too late.
The toilet was positioned above the battery bank. As the saltwater cascaded onto the batteries, it created chlorine gas that rapidly spread through the sub and threatened to kill the crew. Schlitt ordered the sub to surface.
The sub reached the surface about 10 miles from the Scottish coast and was quickly spotted by British planes. One sailor was killed as the sub was attacked. The order was given to scuttle the ship and escape. Three more sailors drowned attempting to make it to shore. The other 37 sailors aboard the U-1206 were quickly captured and became prisoners of war.
Luckily for them, the war was nearly over. The sub sank April 14, 1945. Hitler killed himself April 30 and Germany surrendered May 8.