In a perfect world, the owner never changes his mind, the engineer never alters his drawings, the contractor never has execution problems, the resident engineer’s decisions are perfect, and Mother Nature behaves herself. That perfect world does not exist.
In the real world, with geometric precision, the forces of owner, engineer, contractor, and Mother Nature combine to make change. The owner changes his mind. The engineer changes his drawings. The contractor changes his means and methods. Mother Nature then changes what the others have missed.