In this new post to his site Mathias Verraes approaches the concept of the DRY principle (Don't Repeat Yourself) as being more about knowledge. He includes two "real world" examples where the business rules can change around you.
“Don’t Repeat Yourself” was never about code. It’s about knowledge. It’s about cohesion. If two pieces of code represent the exact same knowledge, they will always change together. Having to change them both is risky: you might forget one of them. On the other hand, if two identical pieces of code represent different knowledge, they will change independently. De-duplicating them introduces risk, because changing the knowledge for one object, might accidentally change it for the other object.
In his examples, he shows how hard-coded rules (like "a product container can only contain 3 products") could just be around certain needs, not the entire range of requests. He covers some of the principles of Domain-Driven Design and how they apply here, pointing out that changing rules in one part of the application can have an effect on other parts depending on it.