Comment 51 for bug 90261

Same results on an upgrade from 8.04 to 8.10 this evening. Quite annoying and mystifying. I am not using any of the aforementioned problem themes.
Theme: Xfce-4.2
Icons: Tango
WM Style: Smallscreen

Terminal output when adding volume control applet:
The program 'xfce4-mixer-plugin' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadAccess (attempt to access private resource denied)'.
  (Details: serial 210 error_code 10 request_code 33 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
** Message: Volume Control: screen changed: 0

** Message: No valid plug window.

(xfce4-panel:6908): Gtk-CRITICAL **: gtk_socket_get_id: assertion `GTK_WIDGET_ANCHORED (socket)' failed

** (xfce4-panel:6908): CRITICAL **: An item was unexpectedly removed: "Volume Control".