FULL PRODUCT VERSION :
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)

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

A DESCRIPTION OF THE PROBLEM :
NullPointerException when executing JTable header updateUI() from MouseListener of this header.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. javac Test.java
2. java Test
3. Click on table header by left mouse button two times.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
nothing
ACTUAL -
NullPointerException

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at javax.swing.plaf.basic.BasicTableHeaderUI$MouseInputHandler.mouseClicked(Unknown Source)
	at java.awt.AWTEventMulticaster.mouseClicked(Unknown Source)
	at java.awt.Component.processMouseEvent(Unknown Source)
	at javax.swing.JComponent.processMouseEvent(Unknown Source)
	at java.awt.Component.processEvent(Unknown Source)
	at java.awt.Container.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Window.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.table.DefaultTableModel;
import javax.swing.table.TableModel;

public class TestTable {
	public static void main(String[] args) {
		TableModel tm = new DefaultTableModel(3, 3);
		final JTable table = new JTable(tm);
		table.getTableHeader().addMouseListener(new MouseAdapter() {
			public void mouseClicked(MouseEvent event) {
				table.getTableHeader().updateUI();
			}
		});
		JScrollPane pan = new JScrollPane(table);
		JFrame fr = new JFrame();
		fr.getContentPane().add(pan);
		fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		fr.setSize(300, 200);
		fr.setVisible(true);
	}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use repaint() instead of updateUI().