The theremin was the first all-electronic musical instrument, played by moving one's hand through the electromagnetic field generated by two oscillators. It was invented by Leon Theremin (Lev Sergeyevich Termen) in 1920. That "accidental" invention set Theremin on a profoundly peculiar life journey.
Leon was a young physicist under the Soviet regime when he accidentally invented his instrument while working on government-sponsored research into proximity sensors. As a Bolshevik, he was honoured when he discovered that Lenin was impressed by the instrument. The Soviet leader actually took lessons from him, and is believed to have had quite a knack for it. Leon was thus sent around the world to showcase the finest of Soviet technology.
Theremin settled in the US, where he patented his theremin, married a prima ballerina from the American Negro Ballet Company, and was suspected of sending American scientific intelligence back to Russia. But Theremin suddenly and mysteriously disappeared in 1938. Read what happened to Theremin and the impact of his later inventions at Messy Messy Chic.