I remember upgrading Ubuntu a few years ago and then having it reboot to a command line error, then spending the rest of the day finding some XML file that was

I remember upgrading Ubuntu a few years ago and then having it reboot to a command line error, then spending the rest of the day finding some XML file that was now malformed and fixing it. That was a ton of fun :-P To be fair, it was kind of my fault. I had made a modification that wasn't quite right, but the new version of something couldn't handle it anymore.