The worst case is millions of dollars spent to build a product only to discover that there are issues late in the testing phase. Wouldn't is be less risky to experiment and validate assumptions earlier in the cycle?
Agile as opposed to methodology X is a bit fuzzy here, as I'm sure testing is extensive and expensive and the feedback cycle is inherently longer, so change is more expensive.
However, it seems that early feedback and validation of design is even more important in these circumstances, since changes late in the product development process and orders of magnitude more expensive than changes early in the process.
Seriously though, in situations where lives are dependent on the solution working correctly, the process that is used to build the solution needs to take into account human error and have rigorous testing.
i.e. I also wouldn't trust a person who "though about and can 'prove'" a solution in Haskell to control that radiation machine without extensive testing.