Unit testing can be this way, but it doesn't have to be. At a really low level, unit testing can be "testing the implementation", which may be useful while you'

Unit testing can be this way, but it doesn't have to be. At a really low level, unit testing can be "testing the implementation", which may be useful while you're writing code, but isn't useful during a refactoring. But slightly higher level tests can be written to almost never have this problem. Let's say you've written code to ensure that a CC transaction doesn't go through with an invalid number. No matter what the implementation looks like - no matter how much you refactor - this test should always work. The same is true for a wide range of useful tests. (These aren't strictly "unit" tests, but they fall into a similar category.) My personal experience is that a good unit test suite is 99% useful when refactoring, and 1% painful.