... are you going to guarantee the absence of the effects of cosmic rays / radiation / jam on your processor?

Actually (BEGIN ANECDOTAL EVIDENCE . . .) I had a software engineering prof in college that used to work on military spec systems. She told a story of doing a demo for some Air Force brass of some sort of avionics that was supposed to be able to work after taking fire. Right before the demo, one of the officers came up and popped out a chip from the board.

She said it still passed . . . :)

So yeah, there are people who engineer things for those type of environments with those kind of constraints.