In this case, the stupid user is me, so I'm not trying to frag anyone :)

Yesterday, we lost power for about an hour. When I brought the old Linux box back up, I couldn't start KDE, and GNOME was acting oddly. My first thought: Oh gosh, the HD finally went (this is an old box, a PII 400 with only 20 GB of disk - running Redhat 7).

It took me until this morning to do what should have been obvious: look at disk space usage. Whoops, 100% of the space in the home directory gone. That's a small problem. Fixed that, and the whole problem went away.