Program testing can be a very effective way to show the presence of bugs, but is hopelessly inadequate for showing their absence.
There are many ways in which a complex system can be broken. In 2000, a Concorde crashed on takeoff from Charles de Gaulle Airport, Paris. The Concorde was a supersonic transport jet, one of the most advanced and complex planes ever built. Some debris left on the runway had flipped up and torn a tire and some of the underside of the hull. ...