Tested Application Step 7: Double the Edges

Find every external dependency in your application and decide, for each, whether it needs a double: third-party requests, email, payments, the clock, randomness. Double them at the boundary, using fakes rather than mocks wherever you can. Keep using your real test database. Then write one test for a failure path you could not otherwise reach: what your application does when the third-party service returns a 500, which is the case nobody tests and everybody eventually meets.