Then there’s Fogbank. A classified material used in nuclear warheads. Produced from 1975 to 1989, then the facility was shut down. When the government needed to reproduce it for a warhead life extension program in 2000, they discovered they couldn’t. A GAO report found that almost all staff with production expertise had retired, died, or left the agency. Few records existed.
After spending an additional $69 million and years of reverse engineering, they finally produced viable Fogbank. Then discovered the new batch was too pure. The original had contained an unintentional impurity that was critical to its function. That fact existed nowhere in any document. Only the workers who made the original batch knew it, and they had retired years earlier.
A nuclear weapons program lost the ability to make a material it invented. The knowledge existed only in people, and the people were gone.