From Conway's Ten Essential Development Practices:
9. Add New Test Cases Before you Start Debugging
"Don't try to fix the problem straight away, though. Instead, immediately add those tests to your test suite. The point is: if the original test suite didn't report this bug, then that test suite was broken. It simply didn't do its job (finding bugs) adequately. Fix the test suite first by adding tests that cause it to fail. Once the test suite is detecting the problem correctly, then you'll be able to tell when you've correctly fixed the actual bug, because the tests will once again fall silent ...
The more thoroughly you test the bug, the more completely you will fix it."