Most of us have seen the Castle Bravo nuclear test at Bikini Atoll, but did you know it was a mistake? Castle Bravo was designed to be a 5 megaton device, but when they set it off it unexpectedly yielded a 15 megaton blast. This was due to a mistake in the design.
The fuel used was lithium 6 and the core was a mixture of 30% lithium 6 and 70% lithium 7, the lithium 7 was thought to be inert. When the detonation occurred the reaction stripped one of the neutrons from the lithium 7 atoms and that turned it in to lithium 6 so instead of a 5 megaton blast what they got was a 15 megaton blast.
For the people at the test site this 15 megaton blast was a terrifying surprise and many of them thought they were going to die right then and there. Some of the people did end up dying later as a result of radiation poisoning.
It seems such an obvious mistake that it's hard to believe that the scientists at the time didn't see it coming.