The value of a program is proportional to the weight of its output. Program complexity always grows until it exceeds the capability of the programmer who must maintain it. If a test installation functions perfectly, all subsequent systems will malfunction. Job control cards that positively cannot be arranged in improper order will be. 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.
Adding manpower to a late software project makes it later. There is always one more bug. It is impossible to make any program foolproof because fools are so ingenious. When things are going well, something will go wrong. When things just can't get any worse, they will.