Quick Answer: Code is "correct by coincidence" when it relies on duplicated, fragile details—like matching magic strings—that just happen to align perfectly to make the application work. Code is "correct by design" when you structurally enforce stability, such as centralizing shared values, ensuring the system cannot easily break from simple typos. You can build an application a hundred different ways. From the outside, the users are happy, the behaviors are exactly what the product manager ...