example of unit tests preventing regression
I am the only developer in my IT shop (a mid-size retailer) writing unit tests.  I have been tasked by management with giving a presentation on the benefits of doing so to our developers.

One of the "aha!" moments that led to my unit-testing addiction was the realization that a unit test suite can prevent regressions in code that you would never have thought to re-test had you been testing manually (because it seemed unrelated to the change you were making).

I am looking for an example of such a test to include in the presentation.  Ideally it would be "real-world" enough to have credibility, but self-contained enough to be appropriate for a classroom setting.  Homebrewed tests are fine as are pointers to relevant texts.  Language of implementation is not important.
example of unit tests preventing regression
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I am the only developer in my IT shop (a mid-size retailer) writing unit tests.  I have been tasked by management with giving a presentation on the benefits of doing so to our developers.

One of the "aha!" moments that led to my unit-testing addiction was the realization that a unit test suite can prevent regressions in code that you would never have thought to re-test had you been testing manually (because it seemed unrelated to the change you were making).

I am looking for an example of such a test to include in the presentation.  Ideally it would be "real-world" enough to have credibility, but self-contained enough to be appropriate for a classroom setting.  Homebrewed tests are fine as are pointers to relevant texts.  Language of implementation is not important.