Lithium hydride
Before the Castle Bravo nuclear weapons test in 1954, it was thought that only the less common isotope lithium-6 would breed tritium when struck with fast neutrons. The Castle Bravo test showed (accidentally) that the more plentiful lithium-7 also does so under extreme conditions, albeit by an endothermic reaction.
Rongelap Atoll
March 1, 1954: United States detonates 15-megaton hydrogen bomb (Castle Bravo test) unaware that fallout will reach Rongelap.