Run the attached TabbedPaneDemo.java testcase on the latest Swing nightly build:
% java -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel TabbedPaneDemo

The testcase will fail to display and throws exceptions like the following:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
       at com.sun.java.swing.plaf.gtk.GTKStyle.get(GTKStyle.java:713)
       at javax.swing.plaf.synth.SynthStyle.getInt(SynthStyle.java:952)
       at javax.swing.plaf.synth.SynthArrowButton$SynthArrowButtonUI.getPreferredSize(SynthArrowButton.java:95)
       at javax.swing.JComponent.getPreferredSize(JComponent.java:1628)
       at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneScrollLayout.layoutContainer(BasicTabbedPaneUI.java:3042)
       at java.awt.Container.layout(Container.java:1432)
       at java.awt.Container.doLayout(Container.java:1421)
       at java.awt.Container.validateTree(Container.java:1519)
       at java.awt.Container.validateTree(Container.java:1526)
       at java.awt.Container.validateTree(Container.java:1526)
       at java.awt.Container.validateTree(Container.java:1526)
       at java.awt.Container.validateTree(Container.java:1526)
       at java.awt.Container.validateTree(Container.java:1526)
       at java.awt.Container.validate(Container.java:1491)
       at java.awt.Window.pack(Window.java:674)
       at TabbedPaneDemo.createAndShowGUI(TabbedPaneDemo.java:104)
       at TabbedPaneDemo.access$000(TabbedPaneDemo.java:21)
       at TabbedPaneDemo$1.run(TabbedPaneDemo.java:113)
       at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
       at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
       at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
       at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
       at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
       at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
       at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
       at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

This is reproducible with Swing's nightly build from 2006-10-20 and later.