I feel the same. I work on a huge meandering system and most of the issues we encounter are caused by the interactions between modules, as you say. I've taken

I feel the same. I work on a huge meandering system and most of the issues we encounter are caused by the interactions between modules, as you say. I've taken to writing and running "end to end" tests which test the whole process end to end and verify the outputs. Wherever it makes sense to add unit tests for specific contained logic, we do that too.