8.Mother nature is a witch. Anything that can go wrong will go wrong. It is impossible to make anything foolproof because fools are so ingenious. Left to themselves, things tend to go from bad to worse. Provided, of course, that you know there is a problem.
Nothing is as easy as it looks.
Everything takes longer than you think. If anything simply cannot go wrong, it will anyway. Whenever you set out to do something, something else must be done first. Every solution breeds new problems. The legibility of a copy is inversely proportional to its importance.