Version:            (using KDE Devel)
Installed from:    Compiled sources

In event.c we can find this:

        case ConfigureRequest:
            {
            if ( e->xconfigurerequest.parent == root ) 
                {
                XWindowChanges wc;
                unsigned int value_mask = 0;
                wc.border_width = 0;
                wc.x = e->xconfigurerequest.x;
                wc.y = e->xconfigurerequest.y;
                wc.width = e->xconfigurerequest.width;
                wc.height = e->xconfigurerequest.height;
                wc.sibling = None;
                wc.stack_mode = Above;
                value_mask = e->xconfigurerequest.value_mask | CWBorderWidth;
                XConfigureWindow( qt_xdisplay(), e->xconfigurerequest.window, value_mask, &wc );
                return true;
                }
            break;
            }

As you can see, sibling gets set to None, but value_mask is not modified. So any application that has CWSibling set (like when using XRestackWindows) will get an X error back, causing many applications to crash.