it’s on Series P, Episode 8, Plants of BBC Qi.
Now known as The Maine Potato Episode.
In early hours of 5th of April 1943 a U.S. destroyer the U.S.S. O’Bannon (DD 450) came across a Japanese sub, the RO-35, in the Pacific. The ship was too close to the boat to lower the guns, so the crew of the U.S.S. O’Bannon started pelting the submarine with potatoes, in the dim light of dawn the Japanese crew who were sleeping on the deck up to that point, had no idea they were potatoes and thought they were grenades so the Japanese ran about frantically getting them off the deck and running for cover. Giving the U.S.S. O’Bannon time to move away, position it’s guns and open fire, sinking the sub.
I love the BBC for bringing stuff like this.