Once a circuit is fabricated, there are many reasons it might not work. In fact, we can guarantee that even the best designs going through the best fabs will sometimes produce defective chips. If we insist on testing every chip for everything that might go wrong, we will spend an eternity testing every chip. If we decide to not test for anything that can go wrong, we will end up with a lot of angry customers with a lot of broken phones. Intelligent design for testability can find a viable middle ground.