I read somewhere that the Apollo Guidance Computer was designed in such a way, that, although it is hugely less powerful than the average smartphone, it was dependable to the point that it never crashed. My questions pertaining to this were:Is designing such a digital system possible, even in the '60's ?What are the constraints that one would want to look out for when attempting to design such a digital system that never crashed, using today's technology? 
How does one go about designing a system that never crashes?
computer architecture