You're working on a new feature and suddenly an old feature stops working even though you wrote unit tests. Or you're refactoring legacy code and you think it's done, but suddenly you find a lot of bugs. This is a common problem that can happen when testing software, where the test results give a false positive about the software, indicating that it's working when it's not. To solve this problem, Test-Driven Development (TDD) is employed, which involves writing tests first and then writing code to make those tests pass. This approach forces developers to see their failing tests and understand how to turn them into green by fixing the problems. TDD turns testing into a design activity, helping to keep code as simple as possible so it's easier to understand and modify. It also provides feedback on both implementation and design quality.