We (authors of this book) almost never debug applications we're working on!
This statement might sound pompous, but it's true. We almost never debug because there is rarely a reason to debug an application. When tests are written before the code and the code coverage is high, we can have high confidence that the application works as expected. This does not mean that applications written using TDD do not have bugs—they do. All applications do. However, when that happens, it is easy to isolate them by simply looking for the code that is not covered by tests.
Tests themselves might not include some cases. In those situations, the action is to write additional tests.