Improving reported cause of WaitTimedOutError will help users.

    public static void main(String[] args) {
        Robot robot = BasicRobot.robotWithCurrentAwtHierarchy();

        final JFrame f = new JFrame("ParentForTwo"); f.setName("ParentForTwo");
        JButton b1 = new JButton("Open dialog 1"); b1.setName("b1");
        b1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JDialog d1 = new JDialog(f, "Dialog 1"); d1.setName("Dialog 1");
                JButton b2 = new JButton("Open dialog 2"); b2.setName("b2");
                b2.addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                        JDialog d2 = new JDialog(f, "Dialog 2"); d2.setName("Dialog 2");
                        d2.getContentPane().add(new JLabel("Just a label"));
                        d2.pack(); d2.setVisible(true);
                    }
                });
                d1.getContentPane().add(b2);
                d1.pack(); d1.setVisible(true);

            }
        });
        f.getContentPane().add(b1);
        f.pack();
        f.setVisible(true);

        FrameFixture ff = WindowFinder.findFrame("ParentForTwo").using(robot);
        ff.button().click();
        ff.dialog().button().click();
        DialogFixture unfindableDialog = ff.dialog("Dialog 2"); //works
        unfindableDialog = ff.dialog(); // fails with a not very helpful error message
                                        // Exception in thread "main" org.fest.swing.exception.WaitTimedOutError: 
                                        // Timed out waiting for dialog to be found using matcher 
                                        // org.fest.swing.core.TypeMatcher[type=java.awt.Dialog, requireShowing=true]
        unfindableDialog.requireFocused();
    }