5.4.1 When things break.

I don't care if you have 1000% code coverage, even NASA with their incredible attention to detail and quality code with tons of peer review and documentation still releases code with unforeseen bugs. So, it's really just a matter of time before things do go wrong. With that in mind your path going forward is simple:

1. Isolate the bug.
2. Write a test for it. It should, obviously, fail.
3. Fix the bug.
4. Rerun the test.
5. Check fixes into the appropriate branch(es).

If this is a bug on your live site you need to create a new branch of the code-base before step one.