gtk_window_set_transient_for() sets window group for the child window (see #312918). It doesn't watch parent's window group later on, but gtk_window_destroy tries to remove the child window from the parent's group. So if parent's group is changed, gtk_widget_destroy(child) yields warning. To reproduce, create a window with a combo box or menu inside (those will create a window), then add this window to some window group. (printing dialog works, if it's transient for the parent).