Traceability
Postscript November 2020: from the days in which I was not allowed to practice "agile"...
You've written requirements. You've created a detailed use cases, and unit tests, and reviewed all three. Code is delivered, passes system testing, moves on to UAT. Whoa, holdup there fella, where's the data validation that prevents the user from doing X? Its right there in the requirements. What happened? Not only was it missed in the unit testing (and possibly use cases), but it was missed in the review process as well.