Code such as this will break if an invalid monitor is passed:

gint
gdk_screen_get_monitor_width_mm	(GdkScreen *screen,
				 gint       monitor_num)
{
  return get_monitor (screen, monitor_num)->width_mm;
}

get_monitor() will return NULL if passed an invalid monitor_num and a segfault will occur.

The following functions in  gdk/x11/gdkscreen-x11.c are effected:
gdk_screen_get_monitor_width_mm()
gdk_screen_get_monitor_height_mm()
gdk_screen_get_monitor_plug_name()
gdk_x11_screen_get_monitor_output()
gdk_screen_get_monitor_geometry()

Is this user not allowed to pass an invalid monitor_num to these functions? If so, this is a pygtk bug.

I originally reported here: http://bugs.gentoo.org/show_bug.cgi?id=284852, the bug is in my older version (2.14.7) too.