Test the Defects

No matter how much you test your code, there will be defects. If your team does its job well, you will find all of your significant defects before you get to production. Regardless of when and by whom the defect is found, writing a test that duplicates the defect and then passes after the fix helps you know that you have fixed the defect and ensures that the defect remains fixed over time.

I prefer to test each defect, at least at the unit level. Every defect, including defects that can be broadly described as integration or interaction problems, trace to one or more defects in a unit of code. Perhaps the caller passes the wrong parameters or invokes functionality in the wrong order. Perhaps the callee does the wrong thing with the arguments or returns the wrong format or value. Maybe the synchronization is handled in a way that allows occasional race conditions. All of these and more can be duplicated and fixed at the unit level.