If your tests are not very thorough you might fall into a false sense of "everything works" just because you tests pass. Theoretically if your tests pass, the code is working; but if we could write code perfectly the first time we wouldn't need tests. The moral here is to make sure to do a sanity check on your own before calling something complete, don't just rely on the tests.
On that note, if your sanity check finds something that is not tested, make sure to go back and write a test for it.
