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.
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. Job control cards that positively cannot be arranged in improper order will be. Interchangeable tapes won't. If the input editor has been designed to reject all bad input, an ingenious idiot will discover a method to get bad data past it. If a test installation functions perfectly, all subsequent systems will malfunction.