The sinking of U-1206 by its own toilet.
U-1206 was a German submarine, fitted with an advanced toilet which was necessary for it to flush when submerged. Due to the complex nature of pressure systems needed to eject the waste, technicians were required to flush.
One day some genius decides that he's got this, tries to flush it himself, and the resulting flood reaches the batteries and releases chlorine gas.
Rather than being killed by poison gas, they decide to surface, whereupon they are promptly located and bombed. All this happened during the U-boat's first active patrol.