I think it's dangerous to rely solely on a unit test suite to guarantee the correctness of your program. Each unit-tested component may pass its own tests, but that doesn't guarantee that by interacting with other (present and future) components it won't exhibit any bug. I guess immutability would go a long way forward in realizing this guarantee, but I've never had the pleasure of working on a software where each component was truly immutable and independent of each other.