If you are that strict on your definition, yes you cannot TDD when fixing a bug. However all TDD advocates will tell you to write the test for the bug before fixing it. If the code was designed with TDD you will not have to refactor the code to write the test (though the test might be much bigger than it would need to be if the code was designed different). You seem to be describing working with legacy code though, not which is a different situation from TDD.