GtkVolumeButton value changes on opening when located near the edge of the screen.
F.e. if the volume button is located near the top of the screen, the volume is at say '0', and you click on the volume button, it pops open, moves the window (with the slider) to stay completely on the screen. This however causes the volume slider itself to change. 
People using my program (gmpc) often experience that clicking the button causes the volume to increase, (quoting a user: causing heart-attacks ear pain and perpetual annoyance).


Other information:
I've added a small example video file..
What you see here is the volume at level 0, I click on the button, if I release directly you see it jumps to (here 64%), if I keep my mouse button pressed it does not move UNTIL I move my mouse, or release the button).

http://download.sarine.nl/gtkvolume-bug.ogv