A dead CMOS battery made my PC act haunted for a week before I worked out what was wrong

For the better part of a month, my home server acted like it was possessed by a ghost. After a power outage, it didn't come back on its own like it usually does, and it happened while I was away from home, resulting in a lot of painful hours working without tools I usually rely on. When I pressed the power button myself, it stalled before Proxmox ever loaded. It sat on a firmware screen waiting for a keypress that a headless machine in the corner was never going to get. When I finally got it running, settings I’d configured a long time ago in the BIOS were completely reset, and when it happened again upon AC power loss, I knew exactly what the issue was.