FULL PRODUCT VERSION :
java version "1.6.0_10-rc"
Java(TM) SE Runtime Environment (build 1.6.0_10-rc-b28)
Java HotSpot(TM) Client VM (build 11.0-b15, mixed mode, sharing)

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

A DESCRIPTION OF THE PROBLEM :
The border or margin for a JToolBar created under Nimbus Look and Feel is different from one created under Metal and then switched to Nimbus. I didn't investigate to find the exact problem, but the apparent border/margin size is different.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run code.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The two dialogs should be the same size.
ACTUAL -
The dialog created under Metal then switched to Nimbus is larger, because the JToolBar it contains has a larger margin or border, or some other layout difference, from the one created under Nimbus.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.*;

public class NimbusToolBarBug2 extends JFrame {
 
   public static void main(String[] args) {
      try {
         UIManager.setLookAndFeel(
               "javax.swing.plaf.metal.MetalLookAndFeel");
      }
      catch (Exception e) {
      }
      final JDialog dg = new JDialog();
      JToolBar tb = new JToolBar();
      tb.add(new JButton("Test"));
      dg.add(tb);
      dg.pack();
      dg.setVisible(true);
   
      SwingUtilities.invokeLater(
         new Runnable() {
            public void run() {
               try {
                  UIManager.setLookAndFeel(
                     "com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
               }
               catch (Exception e) {
               }
               SwingUtilities.updateComponentTreeUI(dg);
               JDialog dg2 = new JDialog();
               JToolBar tb = new JToolBar();
               tb.add(new JButton("Test"));
               dg2.add(tb);
               dg2.pack();
               dg2.setVisible(true);
            }
         });
   }
}

---------- END SOURCE ----------