If a test installation functions perfectly, all subsequent systems will malfunction. Not until a program has been in production for at least six months will the most harmful error be discovered. Job control cards that positively cannot be arranged in proper 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.
Profanity is the one language all programmers know best.
If builders built buildings the way programmers wrote programs, then the first woodpecker that came along would destroy society as we know it. Corollary - An expert is a person who avoids the small errors while sweeping on to the Grand Fallacy.