[lowercased]

> With the problem established, the interviewer pointed out that there was a unit test associated with the bad commit, and it was passing, which seemed like a problem. How could we ensure that this problem didn't reoccur in some later commit?

It would seem that the unit test itself should be replaced with something else, or removed altogether, in addition to whatever structural changes you put in place. If you changed db constraints, I could see, maybe, a test that verifies the constraints works to prevent the previous data flow from being accepted at all - failing with an expected exception or similar. But that may not be what they were wanting to hear?