So the developer decides to do TDD and write a test, but he or she notices the test that’s already there. Now the quick thing to do would be to assume that the QA ticket is right and update the existing test. But that’s not the responsible thing to do. If a test is there, indicating that a 14 digit account number should have 8 asterisks, somebody must have desired that behavior. Before changing the test and the code, the developer should find out who wrote the test. Does somebody else think it should be 8 asterisks? Does one part of the application always want 8, while another wants 10?

Dan Manges’s Blog - Increasing Code Coverage May Be Harmful