One of my favourite perls (:-)) of wisdom was one of the first things a formal methods lecturer said to us and it still stands today - he said "Software is the only product guaranteed not to be fully working at the point of delivery"

I also like the quote from Robinson Crusoe that seems so pertinent to software (mis)management these days: "Now I saw, tho' too late, the folly of beginning a work before we count the cost and before we judge rightly of our strength to go thro' with it".