http://forums.java.net/jive/thread.jspa?messageID=121703#121703

BasicInternalFrameUI.installDefaults() fails with a ClassCastException during a UI switch if the content pane is not a JComponent. Compile and run the attached, then wait for a second for the ClassCastException during the automatic UI switch:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class InternalFrameDemo2 extends JFrame {
    
    JDesktopPane desktop;
    
    public InternalFrameDemo2() {
        super("InternalFrameDemo");
        setBounds(50, 50, 800, 600);
        desktop = new JDesktopPane();
        createFrame();
        setContentPane(desktop);
    }
    
    protected void createFrame() {
        final JInternalFrame internalFrame = new JInternalFrame("Test bug");
        internalFrame.setSize(500,300);
        internalFrame.setLocation(30,30);
        internalFrame.setVisible(true);
        Container c = new Container();
        c.setLayout(new BorderLayout());
        internalFrame.setContentPane(c);
        desktop.add(internalFrame);
        try {
            internalFrame.setSelected(true);
        } catch (java.beans.PropertyVetoException e) {}
        ActionListener al = new ActionListener(){
            public void actionPerformed(ActionEvent e){
               SwingUtilities.updateComponentTreeUI(internalFrame);
            }
        };
        Timer t = new Timer(2000, al);
        t.start();
    }
    
    private static void createAndShowGUI() {
        JFrame.setDefaultLookAndFeelDecorated(true);
        InternalFrameDemo2 frame = new InternalFrameDemo2();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
    
    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}