The following exception was thrown upon dismissing a modal dialog.
I used the mustang nightly build on 4/25 on XP/IE to reproduce the problem.

To reproduce, load the following applet into the IE browser:
http://javaweb.sfbay/~ccheung/Yokogawa/Suspicious/TC05-01_Top.html

Then click on the applet's button.
Wait until a modal dialog and a new window shows up.
Click on the "Ok" button on the modal dialog and you'll see the following exception at the java console.

I think this is caused by the fix to the following bug:
6411322 : Exception thrown inside the destroy/stop makes browser to hang if exception property is set to tru

Exception in thread "AWT-EventQueue-2" java.util.EmptyStackException
at java.util.Stack.peek(Unknown Source)
at java.util.Stack.pop(Unknown Source)
at sun.plugin.viewer.frame.IExplorerEmbeddedFrame.modalityPopped(Unknown Source)
at sun.awt.SunToolkit$ModalityListenerList.modalityPopped(Unknown Source)
at sun.awt.ModalityEvent.dispatch(Unknown Source)
at sun.awt.SunToolkit.notifyModalityChange(Unknown Source)
at sun.awt.SunToolkit.notifyModalityPopped(Unknown Source)
at java.awt.Dialog.modalityPopped(Unknown Source)
at java.awt.Dialog.show(Unknown Source)
at javax.swing.JOptionPane.showOptionDialog(Unknown Source)
at SampleApplet.jButton1_actionPerformed(SampleApplet.java:66)
at SampleApplet$1.actionPerformed(SampleApplet.java:27)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(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.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)