That's often true of tests that were only used to check the validity of code, not its design. In many uses of TDD, you will use somewhat broader specifications

That's often true of tests that were only used to check the validity of code, not its design. In many uses of TDD, you will use somewhat broader specifications to test things. "The login system accepts any user with a valid name by returning '1' and returns '0' if the password isn't the right one". You can have a test to verify that without using special corner cases, and a test such as this one will likely look like any other normal use of your library or function. If you develop all your requirements using TDD, then both the corner cases and the standard cases should be sitting in the test suite. The standard cases can act as documentation and a design tool while corner cases are really more there to insure things work absolutely right.