In the second weekend of December 1988, there was engineering work taking place on the busy main line out of London Waterloo, part of a resignalling project. A mistake was made; a redundant section of wiring was left in place without being properly disconnected or isolated.
Because of this rogue piece of wire, a signal showed green when it should have showed red, at the height of the Monday morning rush hour. A packed commuter train ran past this wrongly-green signal and collided at speed with the train in front. The Clapham Junction rail crash killed 35 people and injured nearly 500.
This is why we test.
This shouldn’t need to be said. But there are still people out there who really ought to know better who insist, despite all evidence, that testing is not a worthwile activity.