Unit test !== only type of test though. Functional tests are a must. I would also say that in this day and age integration tests are also a must. I don’t think

Unit test !== only type of test though. Functional tests are a must. I would also say that in this day and age integration tests are also a must. I don’t think there is any domain now where you are commonly interfacing with only your code end to end, some level of integration is happening and that should be tested too. Nothing’s perfect because we are human, I will say though that having proper functional, unit, and integration tests for a codebase I don’t see this issue crop often and when it does it becomes apparent, and then you write a test for that and then you have coverage for that etc. Software is an organism composed of changing structures, after all.