The Soviet H-Bomb.
I remember reading a quote from some physicist years ago who said that the design of the H-bomb was so clever that only two people in the world could have possibly thought of it. He added that it was quite coincidental that one happened to be American and the other Soviet. It turns out that maybe only one person in the world could have designed it.
"Soviets Stole Bomb Idea From U.S., Book Says."