Start gnome-volume-control. Change the volume by _other_ means. Observe gnome-volume-control fail to notice. Get halfway through filing a bug about the mixer_applet not responding to the mouse wheel any more before noticing that it _is_ in fact working, and it's just that gnome-volume-control isn't showing the changes :)
Moved to http://bugzilla.gnome.org/show_bug.cgi?id=94726
Upstream