[I don't know which component is responsible for the UI for changing
volume/brightness/etc; I guessed gnome-shell, but I don't know if it's
that, gnome-settings-daemon, or something else.]

Several times lately I've changed audio volume with the volume hotkeys,
and the UI popped up with the wrong icon: instead of volume, it shows
the brightness icon, or the microphone icon, or similar. The hotkeys still
change the audio volume; it's just an issue with the UI icon.

Whenever this happens, it continues to happen as long as I hit the
volume hotkeys. Even if I wait and let the UI disappear, and then hit
the volume hotkeys again, the UI will come back up with the wrong icon.
But as soon as I hit a different hotkey that uses a different icon (e.g.
mic mute or brightness), the next time I use the volume keys they'll
have the right icon again.

I managed to capture a screenshot (attached) from using the volume
hotkeys and seeing the brightness icon.

- Josh Triplett