[Roofduck]

I agree that regression scenarios are difficult to identify, this is why its good to have unit tests to begin with.

You're only unit testing the code how you 'intended' for it to work at that time. Even though the tests are written, it probably wouldn't be uncommon for a bug to slip through when running your code, what you then can do is write another test to account for that scenario, then repeat and your code becomes more robust as a result.