I was testing my UPS (APC 550), running NAS4Free 847. I unplugged it from AC power and it ran fine. I changed a UPS setting through the WebGUI and clicked "Save and Restart" so it restarted the UPS daemon with the new settings. The server hardlocked. A simple reboot did not help and I didn't have a monitor connected at the time. I hooked it up to a monitor and I had to ctrl-C out of three or four errors during the next boot. It seemed to run fine after that, the next reboot was successful and it's been fine since, as far as I can tell.
I can provide logs if I did not give enough detail here. Just let me know!