this can be limited though. I just went through refactoring a whole set of classes that implement the same interface, but it was not defined as an Interface. no

this can be limited though. I just went through refactoring a whole set of classes that implement the same interface, but it was not defined as an Interface. now any new class that implements this _must_ meet at least some expectations, enforced by the code. So in one way, OOP is great for limiting side effects.