I guess what meant here is that one should never make mistake of assuming that a highly reliable system can be built. As you start to approach near 100% reliable system, you start experiencing failures that are caused by minute disturbances/flaws in underlying dependencies(hardware, physical location) which can't be controlled. This is what they realized while trying to push the limits to build highly reliable system.
> I guess what meant here is that one should never make mistake of assuming that a highly reliable system can be built.
Wrong guess imho. It means building highly reliable systems requires knowledge and experiences. Trying to build them and solving the problems step-by-step is one way to understand how it can be achieved.
