There are many, of course, but this one is somewhat recent. The story isn't long but finding/fixing it was certainly infuriating.

For some reason xenial's nut-server package is missing "/lib/systemd/system-shutdown/nutshutdown"

I had to set-up the ups w/ a newer Ubuntu release, test and trace using "upsmon -c fsd". And then, after comparing filesystems, realize the file was missing from xenial and simply copy it over.

It took much longer than I'd expect due to the breadth of nut, not knowing if my hardware or conf settings were to blame (they weren't), and the expectation that things should "just work".

I know, I probably shouldn't have a machine still running xenial :-P Alas, all is well now, and I've kissed apcupsd goodbye.