Peter Bright at Ars Technica had an interesting article on the Windows development process, with a tidbit of information I thought was interesting. Apparently it's permitted to integrate code into Windows that doesn't have tests:
I expect there are automated tests, but it's been my experience that automated tests are usually not very deeply implemented. They often test the expected code paths, and new code, with new tests that fail to review the existing tests can often result in regressions that are not caught.