Between each step of our refactoring, all our unit tests pass, but the end-to-end tests don't. This situtation is possible when the components and their unit tests are so well decoupled (due to using message passing) that changing one component does not affect other components - until they are plugged together. We use the end-to-end tests to drive the refactoring, by fixing components in the order that they fail, until the refactoring has been done to all of them.
