If at first you don't succeed, you will never succeed. 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.
Any system which relies on human reliability is unreliable. The only difference 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 inherent unreliability of the system in which they are used. The error detection and correction capabilities of a system will serve as the key to understanding the types of error which they cannot handle.