Because the testable specification didn't follow unit testing discipline, it didn't satisfy my need for unit tests. I had to write additional tests to fill that need. It's fine to do that, as long as I don't skimp on the real unit tests.
In no way. It's actually desirable to run your tests as often as possible.
You've lost the opportunity to give your code's planned interface a test drive before you set it in stone. You've lost the opportunity to write down your expectations without having them contaminated by the actual behavior of your first implementation. You've lost the opportunity to have the computer tell you what needs to be done to create a working implementation.
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