Late for the "coolest bug" party. But: I helped migrate an organization from proprietary software to FOSS and they found a bug. They were used to being "the only ones who ever encountered this problem" (along with a few hundred other customers). Now we sat down and had a look at the code. Found out that the Perl script didn't pass the value in question to the template. An easy fix but it was an eye opening moment for them of the benefits of the FOSS path.