If your code has been "running without a hitch" and "an old project" (which I assume means you are no longer actively updating it) why are you adding tests?
Out of curiosity, what's the ROI that you see in adding tests to that project?
Out of curiosity, what's the ROI that you see in adding tests to that project?
Joking aside, in order to effectively prevent that, a test suite must be in place to prevent regressions when you change/add code. If there isn't one, you need to decide between cranky clients or the nastyness of adding test to legacy code.
Another sad truth to consider is that just the process of refactoring your code to add tests can just as easily break functionality. I've learned this the hard way.
I believe __abc was basing his comment off the assumption that the code wasn't being actively developed, which significantly reduces the ROI of adding tests to an existing project.
Sounds like it's more back to the OP's response below, that it's an exercise in learning.