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.
Castle Koon
The Koon shot of Operation Castle was a test of a University of California Radiation Laboratory (UCRL), now Lawrence Livermore National Laboratory (LLNL) designed thermonuclear device.