To err is human, but to really screw things up requires a computer. If you put tomfoolery into a computer, nothing comes back out but tomfoolery. But this tomfoolery, having passed through a very expensive machine, is somehow ennobled, and no one dares to criticize it. An expert is a person who avoids the small errors while sweeping on to the Grand Fallacy. Glib's Laws of Reliability 1.
Computers are unreliable, but humans are even more unreliable.
At the source of every error which is blamed on the computer you will find at least two human errors, including the error of blaming it on the computer. Any system which relies on human reliability is unreliable. The only diffrence between the fools and the criminal who attacks a system is that the fool attacks unpredictably and on a broader front. A system tends to grow in terms of complexity rather than simplification, until the resulting unreliability becomes intolerable. Self-checking systems tend to have a complexity in proportion to the inherant unreliability of the system in which they are used.