Anecdotal: I once helped out a team who was writing a Maven plugin for doing static tests on some js code during build. There was already a test suite with a bunch of test code. As my stuff was fairly complicated and I have a habit of writing unit tests for such I added a bunch. Fast forward a year and a half later: I was greeted with a mail that there was a bug in it. I had to fight the better part of a day to nail it down: First not being familiar any longer with the code and secondly because a bunch of stuff has been added meanwhile. I fixed and thought it was a good idea to add a test as it was a nasty corner case. I headed for the natural place where the test would go and found -- exactly the test I was going to write, nicely commented out. A quick check with Git revealed that I added this test initially, which then was commented out when the new feature causing the bug was added. Firing up git blame was next... This is why I am fond of having tests: you are stomped onto it if you break something at least if your test suite is worth its salt.