The test iterates over sets of data, in particular - various GraphicsConfigurations.
The following example fails on 6u5 (but passes on 6u10b13), both WinXP and Vista:

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

public class TheBug {

   public static void main(String[] args) {
       Dialog parent = new Dialog((Frame) null);
       for (GraphicsConfiguration gc : getGCs()) {
           JDialog dialog = new JDialog(parent, "", false, gc);
           if (gc == null) {
               Window owner = dialog.getOwner();
               GraphicsConfiguration expected = owner.getGraphicsConfiguration();
               GraphicsConfiguration actual = dialog.getGraphicsConfiguration();
               /** quote from JDialog(Dialog owner, String title, boolean modal, GraphicsConfiguration gc) {
                * If <code>gc</code> is <code>null</code>, the same
                * <code>GraphicsConfiguration</code> as the owning Dialog is used.
                */
               if (expected != actual) {
                   throw new RuntimeException("Not same. Expected: " + expected + ", actual: " + actual);
               }
           }
       }
   }

   public static ArrayList<GraphicsConfiguration> getGCs() {
       ArrayList<GraphicsConfiguration> result = new ArrayList<GraphicsConfiguration>();
       result.add(null);
       GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
       GraphicsDevice[] devices = ge.getScreenDevices();
       for (GraphicsDevice device : devices) {
           GraphicsConfiguration[] gcs = device.getConfigurations();
           result.addAll(Arrays.asList(gcs));
       }
       return result;
   }
}

The message is like this:
Not same. Expected: sun.awt.Win32GraphicsConfig@1270b73[dev=Win32GraphicsDevice[screen=0],pixfmt=0], actual: sun.awt.Win32GraphicsConfig@156ee8e[dev=Win32GraphicsDevice[screen=0],pixfmt=1]