I suspect rbanffy is talking about things like avionics, medical device control, and the space shuttle.  This is a whole other ball game, and it <i>is</i> possible to ship software that is pretty darn close to perfect in those domains.  They cost a few orders of magnitude more per requirement, and the requirements are sharply limited.  These systems usually operate on a very limited number of input values and internal states, so you can often simply enumerate all the possible inputs and all the possible states to verify that the system works as designed.