I have seen some very very bad tests written with mocks. The tests were written in such a way that you couldn't change the underlying implementation without com

I have seen some very very bad tests written with mocks. The tests were written in such a way that you couldn't change the underlying implementation without completely rewriting them because they were testing underlying implementation behavior down to ensuring that certain methods were called on certain libraries used by the underlying implementation code. Please don't do this. It is a nightmare to improve the implementation.