The tests passed. What didn't we check?

The developers have written tests, everything is green.

What should QA do now – repeat the same checks? I propose a different approach: look not at the number of tests, but at what important system properties they actually test and what remains outside the tests.

We'll explore how to turn a simple requirement into several testable assertions, how to analyze developers' tests and find missed states, errors at the interfaces of systems, crashes, and other blind spots. We'll also look at how to recognize after a release that an important system property has been violated.

Finally, we'll put together a simple algorithm that can be used during a regular review of a task or code changes.