Alexander calculated an estimate of the mistakes in a buliding, as mistakes per unit volume. As you are building, you make N decisions per hour; it takes you about a year to build a house, so you can compute the number of possible mistakes. Bad decisions are cumulative.
A house with a living room that is always shady and glum resulted from a mistake in the beginning: not putting the living room on the sunny side. A staircase where the last step is not the same height as the others resulted from a measurement mistake (in the real world, not on paper plans).
So how do we avoid the big mistakes, and how do we minimize the small mistakes?