The other day, I awoke to find most of my machines had rebooted, although not all did. This indicates a brown-out or power flux. I had to intervene because the machines didn't come up cleanly, as some hung at the BIOS checks.
I've fixed one...it wasn't detecting the DVD burner and I don't know why. The other is an old box that has an intrusion detection feature that continues to tell me that the case was recently opened.
Lastly, my Slackware box would NOT boot cleanly, as somehow the filesystem got borked. I had to manually check the filesystem and repair it before it would boot cleanly...this took a few days. Until then, this box was officially down.
I think it's time to invest in a UPS.