If you perceive that there are four possible ways in which something can go wrong, and circumvent these, then a fifth way, unprepared for, will promptly develop. Left to themselves, things tend to go from bad to worse. If everything seems to be going well, you have obviously overlooked something. Nature always sides with the hidden flaw. Mother nature is a bitch.
Computers are unreliable, but humans are even more unreliable.
Any system that depends upon human reliability is unreliable. Undetectable errors are infinite in variety, in contrast to detectable errors, which by definition are limited. Investments in reliability will increase until it exceeds the probable cost of errors, or until someone insists on getting some useful work done. TOP Profanity is the one language understood by all programmers. Not until a program has been in production for six months will the most harmful error be discovered.