As we continue to learn from the XP book, Extreme Programming Explained, today we discuss what it means for software systems to go sour and how to prevent this from happening. We talk about what happens when the cost of making changes or defects rises so much that the system must be replaced and how XP creates and maintains a comprehensive suite of tests to counteract this risk. Tune in to learn more about how pair programming can help prevent the system from going sour, what to do when you are writing legacy code, the types of tests you should write and focus on, and so much more!