We create specs that assert incorrect results, then fix them based on the actual test behavior. Our goal is to capture the current behavior as completely as possible so that we’ll immediately know if code changes break the current behavior, so we aim for 100% C0 coverage (even though that’s no guarantee of bug-freedom!), which is challenging because the code as presented has no seams. Our effort results in finding a bug that crippled thousands of Microsoft Zune players on December 31, 2008.

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