I used to work for a company that produced avionics software. Obviously, we had to write solid code.
The On-Board Shuttle group is (was) even more bad-ass than that. For them, 5 9s (99.999% failure rate) was unacceptably lax.
But how much work the software does is not what makes it remarkable. What makes it remarkable is how well the software works. This software never crashes. It never needs to be re-booted. This software is bug-free. It is perfect, as perfect as human beings have achieved. Consider these stats : the last three versions of the program -- each 420,000 lines long-had just one error each. The last 11 versions of this software had a total of 17 errors. Commercial programs of equivalent complexity would have 5,000 errors.