Golub's Laws of Computerdom 1. Fuzzy project objectives are used to avoid embarrassment of estimating the corresponding costs. The effort required to correct course increases geometrically with time. Project teams detest weekly progress reporting because it so vividly manifests their lack of progress. Gilb's Laws of Unreliability 1.
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. Investment in reliability will increase until it exceeds the probable cost of errors, or until someone insists on getting some useful work done. There's always one more bug. Troutman's Postulate 1.