Null Oddity
Inexplicable oddity of the day: X was hanging after log in. It looked like it got stuck on xrdb.
Inexplicable solution: /dev/null was not readable. (Not only do I not know why /dev/null being unreadable caused the problem, I also don’t know why /dev/null lost its world-readable bit).
There’s a first time for everything, I guess, and this is the first time /dev/null has been the crucial point in fixing my system.
I’m not sure I’d ever be able to explain how to diagnose and fix problems like this to newbies.