In a curious instance of software misbehavior, my desktop now thinks the "Media Stop" button on my Saitek Eclipse II is, in fact, another "Volume Down" key. The two keys are different keycodes (174 and 122, respectively), but apparently are both getting mapped to keysym 0x1008ff11.
I was going to put off investigating this until I realized that I still have an old (and broken!) ~/.Xmodmap. Mystery solved.
I got spam in the mail today from the "church" of scientology. Weird.
Also arriving in the mail today was something very much not spam: my ACLU membership card. Yay!