You can’t change or reorganize the behavior of the real thing without changing everything that using it all at once. This is challenging enough without elaborate handwritten mocks, but much worse with them. Most people don’t have the sort of stubbornness it takes to overcome this sort of adversity. So you’ve locked in your tech debt even harder.

If you have one function simultaneously using large parts of another chunk of code, such that you feel like you should write logic instead of stubs to simulate it, you already have a huge coupling problem that you should fix, instead of shoveling more code after bad. That’s what I mean by “it should hurt.” The friction is not a bug, it’s a feature. Slow your roll and look at your busted architecture, instead of cementing it in place with a layer of tests.

The other difficulty with mutating your mocks or otherwise changing testing tools is with negative tests. The changes can and sadly do end up creating tests that can’t fail (evergreen) but still increase coverage and confidence. Like a safety railing that has corroded, or a broken smoke detector. Simple mocks that exist entirely within the test, or the suite at farthest, are more amenable to change. Our job is change, when you get right down to it.