Some of our older tests at work are an absolute nightmare to deal with. We went through a phase where we basically just mocked everything non-trivial and ended up with a rewording of the code itself. Any time you make a change to that code, even if it still behaves exactly the same from a user point of view, the tests blow up into pieces and you end up having to rewrite or throw out most of them.

We moved to a behavior-focused testing style, and it has been much more robust to non-functional changes and refactorings in the code.