FULL PRODUCT VERSION :
java version "1.6.0-beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-beta-b59g)
Java HotSpot(TM) Client VM (build 1.6.0-beta-b59g, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
When a JPanel  with GridbagLayout as its LayoutManager  is added to a JFrame, or A JFrames LayoutManager is set to GridBagLayout an ArrayIndexOutOfBoundsException is thrown.

I have not checked this with any other containers.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run source code example

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
JFrame will show
ACTUAL -
ArrayIndexOutOfBoundsException is thrown

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
	at java.lang.System.arraycopy(Native Method)
	at java.awt.GridBagLayout.GetLayoutInfo(Unknown Source)
	at java.awt.GridBagLayout.getLayoutInfo(Unknown Source)
	at java.awt.GridBagLayout.preferredLayoutSize(Unknown Source)
	at java.awt.Container.preferredSize(Unknown Source)
	at java.awt.Container.getPreferredSize(Unknown Source)
	at javax.swing.JComponent.getPreferredSize(Unknown Source)
	at javax.swing.JRootPane$RootLayout.preferredLayoutSize(Unknown Source)
	at java.awt.Container.preferredSize(Unknown Source)
	at java.awt.Container.getPreferredSize(Unknown Source)
	at javax.swing.JComponent.getPreferredSize(Unknown Source)
	at java.awt.BorderLayout.preferredLayoutSize(Unknown Source)
	at java.awt.Container.preferredSize(Unknown Source)
	at java.awt.Container.getPreferredSize(Unknown Source)
	at java.awt.Window.pack(Unknown Source)
	at com.spectrum.genius.test.NewJFrame.<init>(NewJFrame.java:36)
	at com.spectrum.genius.test.NewJFrame.main(NewJFrame.java:23)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
#Test Case 1
import java.awt.GridBagLayout;

import javax.swing.WindowConstants;

public class NewJFrame extends javax.swing.JFrame{

    public static void main(String[] args){
        NewJFrame inst = new NewJFrame();
        inst.setVisible(true);
    }

    public NewJFrame() {
        super();
        GridBagLayout thisLayout = new GridBagLayout();
        thisLayout.rowWeights = new double[] { 0.1, 0.1, 0.1, 0.1 };
        thisLayout.rowHeights = new int[] { 7, 7, 7, 7 };
        thisLayout.columnWeights = new double[] { 0.1, 0.1, 0.1, 0.1 };
        thisLayout.columnWidths = new int[] { 7, 7, 7, 7 };
        getContentPane().setLayout(thisLayout);
        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        pack();
        setSize(400, 300);
    }

}

-------------------------------------------------------------------------------------------------------
#Test Case 2
import java.awt.GridBagLayout;

import javax.swing.JPanel;
import javax.swing.WindowConstants;

public class NewJFrame extends javax.swing.JFrame{

    public static void main(String[] args){
        NewJFrame inst = new NewJFrame();
        inst.setVisible(true);
    }

    public NewJFrame() {
        super();
        GridBagLayout thisLayout = new GridBagLayout();
        thisLayout.rowWeights = new double[] { 0.1, 0.1, 0.1, 0.1 };
        thisLayout.rowHeights = new int[] { 7, 7, 7, 7 };
        thisLayout.columnWeights = new double[] { 0.1, 0.1, 0.1, 0.1 };
        thisLayout.columnWidths = new int[] { 7, 7, 7, 7 };
        JPanel p = new JPanel(thisLayout);
        setContentPane(p);
        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        pack();
        setSize(400, 300);
    }

}
-------------------------------------------------------------------------------------------------------
#Test Case 3
import java.awt.GridBagLayout;

import javax.swing.JPanel;
import javax.swing.WindowConstants;

public class NewJFrame extends javax.swing.JFrame{

    public static void main(String[] args){
        NewJFrame inst = new NewJFrame();
        inst.setVisible(true);
    }

    public NewJFrame() {
        super();
        GridBagLayout thisLayout = new GridBagLayout();
        thisLayout.rowWeights = new double[] { 0.1, 0.1, 0.1, 0.1 };
        thisLayout.rowHeights = new int[] { 7, 7, 7, 7 };
        thisLayout.columnWeights = new double[] { 0.1, 0.1, 0.1, 0.1 };
        thisLayout.columnWidths = new int[] { 7, 7, 7, 7 };
        JPanel p = new JPanel(thisLayout);
        getContentPane().add(p);
        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        pack();
        setSize(400, 300);
    }

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use a JDialog instead of a JFrame

Release Regression From : 5.0u6
The above release value was the last known release where this 
bug was known to work. Since then there has been a regression.