Which is why comprehensive testing is important. If we have test cases that accurately capture our requirements, then we can refactor and know when we've err'd from the path. If we don't have test cases, then we have to make best guesses (and we know how that one turns out).
Accuracy is key, here. I came into my current job with some pretty awful tests in place that “passed” and showed good coverage but did absolutely nothing for actually hitting the necessary cases...
