On the essentials of software having an update to date test suite

Mike Gunderloy, from his book Rails Rescue Handbook
(from the sample chapter on Tests)

5. Testing Rescues

There are two possible situations when you look at the tests in an application that’s come to you for rescue:
1. There are no tests
2. There are bad tests
Oh, I suppose it’s possible that some day there will be an application that needs to be rehabilitated despite having
an excellent and up-to-date test suite – but I’ve never seen it. More typically, there are no tests at all, or tests that
haven’t been maintained and that are falling into disarray.

emphasis mine.

What this means

If you ever want to add something to your application, you need to keep a test-suite written and running as you go.

Tested apps don’t end up in the hospital.

Tested apps last and last and last.  If an untested app lasts, it lasts only through many tears and weeping.