Some things change, others stay the same. You change the system, and now some part of the application works differently. The way for you to determine if it's e

Some things change, others stay the same. You change the system, and now some part of the application works differently. The way for you to determine if it's expected or not is through the spec of the system. Either it's a bug to have changed it or it wasn't. In theory, you can use stuff like formal methods while changing a system when you come across things that you think _shouldn't_ change. It's the same concept as tests, really. In practice I have not found any good tooling for this in production systems. The best alternatives are using types to enforce some rules, and to make APIs that make expressing invalid state impossible. But this is only a small subset of the kind of guarantees you want to have.