"You should only test code you wrote that can break. " A common source of errors are mismatches between what your dependencies say they'll do and what they'll

"You should only test code you wrote that can break. " A common source of errors are mismatches between what your dependencies say they'll do and what they'll actually do. There's been huge, costly failures from developers expecting the dependency would honor its documentation or just take a standard approach to error handling. So, either testing them or at least good catch-all error handling + logging should be done.