I was recently having some weird intermittent problems where one server in my mini-rack was mysteriously hanging, usually waiting until I was out of town; the problems went away as soon as I replaced the very old batteries in the UPS, so presumably it was getting a brownout during the automatic testing cycle or something.
I used to work for a large music streaming provider and we had thousands and thousand of servers. The data center would put out a notice that they're going to do UPS maintenance or Switchgear maintenance... and usually that night we would have maybe up to 20 servers have issues. Hard hang or maybe a reboot. Meanwhile everything looked okay. I'd imagine if you watched the sine wave under a microscope there was some kind of glitch when the transfer happened, but not enough to effect most of the systems. Pretty wild. - Ethan