After setting GOBJECT_DEBUG=objects comparing the outputs
of the following program run with ITERATIONS set to 2 and 1:

#include <gtk/gtk.h>    

#ifndef ITERATIONS
#define ITERATIONS 2
#endif

int
main (int argc,
      char *argv[])     
{
  if (gtk_init_check (&argc, &argv))
    {
      int i;

      for (i = 0; i < ITERATIONS; ++i)
        {
          GtkWidget *win = gtk_window_new (GTK_WINDOW_TOPLEVEL);
          GtkWidget *vol = gtk_volume_button_new ();

          gtk_container_add (GTK_CONTAINER (win), vol);
          gtk_widget_show_all (win);
          gtk_widget_destroy (win);
        }
    }

  return 0;
}
      

the number of objects alive at exit increases for the presence
of a GtkAdjustment.
      
The problem is that GtkScaleButton tries to manage (set/get) an adjustment
that is declared in a ui file as a template child. 
The reference added in init:

https://git.gnome.org/browse/gtk+/tree/gtk/gtkscalebutton.c?h=gtk-3-22#n365

would be balanced in finalize by this unref:

https://git.gnome.org/browse/gtk+/tree/gtk/gtkscalebutton.c?h=gtk-3-22#n469

but because priv->adjustment is registered as the automated child pointer

https://git.gnome.org/browse/gtk+/tree/gtk/gtkscalebutton.c?h=gtk-3-22#n337

it gets NULL-ed during GObjectClass.dispose() and so the unref is skipped.

https://developer.gnome.org/gtk3/stable/GtkWidget.html#gtk-widget-class-bind-template-child-full