Is this TDD? (When you can't do: Red-Green-Refactor)

I've been thinking a lot about testing recently.

Specifically, I've been thinking about using TDD and legacy code. (Yes, I've got Michael Feathers's book and will need to reread it. This is probably covered there.)

So, I've just received a bug report that I decide to work on and fix:

1. I created some tests that recreate the issue. Running them verified that they reproduced the issue (they failed) so my tests were RED.
2. I then changed the code so the tests passed. Now my tests were all GREEN.
3. Then I tidied up the code a bit more. (I REFACTORED the code.)
4. Finally, I ran the tests again to make sure they all still pass. They did.
   
   
5. But then I thought of another edge case that wasn't currently tested but probably should be.
6. So, I create a test. And, it passes!

Oh, no! It didn't go Red first, so I can't be doing "proper TDD"™.

I'm happy that the code does correctly handle (and document) this test case but how should I test it "properly"?

• Change the code so the test fails? and then change it back?
• Change the test so it fails? and then change it back?
• Just leave it?
• Something else?

What would you do?

BTW. I'm currently planning a book all about testing. It will focus more on the concepts than the details but I think I can bring some new ideas to the subject. (Despite what you may think by me asking this question.) If you want to know more, be sure to sign up for my (very occasional) newsletter.