Tsar Bomba
There existed a common (but entirely false) myth that the Tsar Bomba was designed per personal order from Nikita Khrushchev, at a meeting on July 10, 1961, with a total research and development time of only 112 days; the actual development of the final stage of the Tsar Bomba (then already placed in KB-11) did indeed take 112 days. In fact, development began in 1956.