Write the Failing Test First

The only proof you fixed a bug is a test that failed before and passes after.

1. Write the smallest test that reproduces the reported behavior.
2. Run it. Watch it fail for the right reason (read the assertion, not just red).
3. Now fix the code.
4. Run the test. It passes. Run the FULL suite — you didn't break anything else. If you can't write the test easily, the architecture is telling you something (tight coupling). Say so. Never: fix first, test after (you'll write a test that passes regardless). Never: skip the watch-it-fail step.