I am using Swing-0.4 on JDK1.1.4.
steps to reproduce:
1) launch swing demo Stylepad
2) hightlight a paragraph
3) select Font->Courier or Times Roman
4) repeat step 2 and 3 a couple of times
4) The Exception thrown

Here is the stack trace:
-------------------------
Exception occurred during event dispatching:
java.lang.NullPointerException
at com.sun.java.swing.text.DefaultStyledDocument$ElementBuffer.canJoin(DefaultStyledDocument.java:885)
at com.sun.java.swing.text.DefaultStyledDocument$ElementBuffer.removeElements(DefaultStyledDocument.java:862)
at com.sun.java.swing.text.DefaultStyledDocument$ElementBuffer.removeElements(DefaultStyledDocument.java:844)
at com.sun.java.swing.text.DefaultStyledDocument$ElementBuffer.removeUpdate(DefaultStyledDocument.java:630)
at com.sun.java.swing.text.DefaultStyledDocument$ElementBuffer.remove(DefaultStyledDocument.java:586)
at com.sun.java.swing.text.DefaultStyledDocument.removeUpdate(DefaultStyledDocument.java:384)
at com.sun.java.swing.text.AbstractDocument.remove(AbstractDocument.java:144)
at com.sun.java.swing.text.JTextComponent.cut(JTextComponent.java:223)
at com.sun.java.swing.text.JTextComponent$CutAction.actionPerformed(JTextComponent.java:963)
at com.sun.java.swing.AbstractButton$2.actionPerformed(AbstractButton.java:579)
at com.sun.java.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:222)
at com.sun.java.swing.AbstractButton.doClick(AbstractButton.java:130)
at com.sun.java.swing.basic.BasicMenuUI$MenuListener.mouseReleased(BasicMenuUI.java:176)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:236)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:235)
atException occurred during event dispatching:
java.lang.StringIndexOutOfBoundsException: String index out of range: -135
at java.lang.String.<init>(String.java)
at com.sun.java.swing.text.Segment.toString(Segment.java:79)
at com.sun.java.swing.text.LabelView.getPreferredShape(LabelView.java:111)
at com.sun.java.swing.text.ParagraphView.determineSpan(ParagraphView.java:479)
at com.sun.java.swing.text.ParagraphView.computeRow(ParagraphView.java:425)
at com.sun.java.swing.text.ParagraphView.layout(ParagraphView.java:401)
at com.sun.java.swing.text.ParagraphView.changedUpdate(ParagraphView.java:233)
at com.sun.java.swing.text.CompositeView.changedUpdate(CompositeView.java:304)
at com.sun.java.swing.plaf.DefaultTextUI$JViewFactory.changedUpdate(DefaultTextUI.java:505)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.changedUpdate(DefaultTextUI.java:813)
at com.sun.java.swing.text.AbstractDocument.externalNotifyChange(AbstractDocument.java:437)
at com.sun.java.swing.text.DefaultStyledDocument.setCharacterAttributes(DefaultStyledDocument.java:247)
at com.sun.java.swing.JTextPane.setCharacterAttributes(JTextPane.java:293)
at com.sun.java.swing.JTextPane$FontFamilyAction.actionPerformed(JTextPane.java:463)
at com.sun.java.swing.AbstractButton$2.actionPerformed(AbstractButton.java:579)
at com.sun.java.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:222)
at com.sun.java.swing.AbstractButton.doClick(AbstractButton.java:130)
at com.sun.java.swing.basic.BasicMenuUI$MenuListener.mouseReleased(BasicMenuUI.java:176)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:236)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:235)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:235)
at java.awt.Component.processMouseEvent(Component.java:2284)
at java.awt.Component.processEvent(Component.java:2129)
at java.awt.Container.processEvent(Container.java:890)
at java.awt.Component.dispatchEventImpl(Component.java:1764)
at java.awt.Container.dispatchEventImpl(Container.java:935)
at java.awt.Component.dispatchEvent(Component.java:1704)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:1539)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:1448)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:1383)
at java.awt.Container.dispatchEventImpl(Container.java:922)
at java.awt.Window.dispatchEventImpl(Window.java:443)
at java.awt.Component.dispatchEvent(Component.java:1704)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:63)
Exception occurred during event dispatching:
java.lang.NullPointerException
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:193)
at com.sun.java.swing.text.ParagraphView.modelToView(ParagraphView.java:135)
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:197)
at com.sun.java.swing.plaf.DefaultTextUI$JViewFactory.modelToView(DefaultTextUI.java:459)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.modelToView(DefaultTextUI.java:696)
at com.sun.java.swing.plaf.DefaultTextUI.modelToView(DefaultTextUI.java:246)
at com.sun.java.swing.text.DefaultHighlighter.renderHighlight(DefaultHighlighter.java:169)
at com.sun.java.swing.text.DefaultHighlighter.paint(DefaultHighlighter.java:58)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JViewport.paint(JViewport.java:193)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.paint(DefaultTextUI.java:593)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JBufferedPane.paint(JBufferedPane.java:238)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JBufferedPane.paint(JBufferedPane.java:238)
at java.awt.Container.paint(Container.java:702)
at java.awt.Component.dispatchEventImpl(Component.java:1723)
at java.awt.Container.dispatchEventImpl(Container.java:935)
at java.awt.Window.dispatchEventImpl(Window.java:443)
at java.awt.Component.dispatchEvent(Component.java:1704)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:63)
Exception occurred during event dispatching:
java.lang.NullPointerException
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:193)
at com.sun.java.swing.text.ParagraphView.modelToView(ParagraphView.java:135)
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:197)
at com.sun.java.swing.plaf.DefaultTextUI$JViewFactory.damageRange(DefaultTextUI.java:478)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.damageRange(DefaultTextUI.java:710)
at com.sun.java.swing.plaf.DefaultTextUI.damageRange(DefaultTextUI.java:271)
at com.sun.java.swing.text.DefaultHighlighter.removeHighlight(DefaultHighlighter.java:117)
at com.sun.java.swing.text.DefaultCaret.setDot(DefaultCaret.java:370)
at com.sun.java.swing.text.JTextComponent.setCaretPosition(JTextComponent.java:327)
at com.sun.java.swing.text.DefaultTextController.positionCursor(DefaultTextController.java:204)
at com.sun.java.swing.text.DefaultTextController.mousePressed(DefaultTextController.java:292)
at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:225)
at java.awt.Component.processMouseEvent(Component.java:2281)
at java.awt.Component.processEvent(Component.java:2129)
at java.awt.Container.processEvent(Container.java:890)
at java.awt.Component.dispatchEventImpl(Component.java:1764)
at java.awt.Container.dispatchEventImpl(Container.java:935)
at java.awt.Component.dispatchEvent(Component.java:1704)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:1539)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:1444)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:1383)
at java.awt.Container.dispatchEventImpl(Container.java:922)
at java.awt.Window.dispatchEventImpl(Window.java:443)
at java.awt.Component.dispatchEvent(Component.java:1704)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:63)
Exception occurred during event dispatching:
java.lang.NullPointerException
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:193)
at com.sun.java.swing.text.ParagraphView.modelToView(ParagraphView.java:135)
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:197)
at com.sun.java.swing.plaf.DefaultTextUI$JViewFactory.modelToView(DefaultTextUI.java:459)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.modelToView(DefaultTextUI.java:696)
at com.sun.java.swing.plaf.DefaultTextUI.modelToView(DefaultTextUI.java:246)
at com.sun.java.swing.text.DefaultHighlighter.renderHighlight(DefaultHighlighter.java:169)
at com.sun.java.swing.text.DefaultHighlighter.paint(DefaultHighlighter.java:58)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JViewport.paint(JViewport.java:193)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.paint(DefaultTextUI.java:593)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JBufferedPane.paint(JBufferedPane.java:238)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JBufferedPane.paint(JBufferedPane.java:238)
at com.sun.java.swing.JComponent._paintImmediately(JComponent.java:2248)
at com.sun.java.swing.JComponent.paintImmediately(JComponent.java:2158)
at com.sun.java.swing.DirtyRegionManager.paintDirtyRegions(DirtyRegionManager.java:277)
at com.sun.java.swing.DirtyRegionManager.actionPerformed(DirtyRegionManager.java:208)
at com.sun.java.swing.Timer$DummyComponent.processEvent(Timer.java:256)
at java.awt.Component.dispatchEventImpl(Component.java:1764)
at java.awt.Component.dispatchEvent(Component.java:1704)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:63)
Exception occurred during event dispatching:
java.lang.NullPointerException
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:193)
at com.sun.java.swing.text.ParagraphView.modelToView(ParagraphView.java:135)
at com.sun.java.swing.text.CompositeView.modelToView(CompositeView.java:197)
at com.sun.java.swing.plaf.DefaultTextUI$JViewFactory.modelToView(DefaultTextUI.java:459)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.modelToView(DefaultTextUI.java:696)
at com.sun.java.swing.plaf.DefaultTextUI.modelToView(DefaultTextUI.java:246)
at com.sun.java.swing.text.DefaultHighlighter.renderHighlight(DefaultHighlighter.java:169)
at com.sun.java.swing.text.DefaultHighlighter.paint(DefaultHighlighter.java:58)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JViewport.paint(JViewport.java:193)
at com.sun.java.swing.plaf.DefaultTextUI$JTextView.paint(DefaultTextUI.java:593)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JBufferedPane.paint(JBufferedPane.java:238)
at java.awt.Container.paint(Container.java:702)
at com.sun.java.swing.JComponent.paint(JComponent.java:207)
at com.sun.java.swing.JBufferedPane.paint(JBufferedPane.java:238)
at com.sun.java.swing.JComponent._paintImmediately(JComponent.java:2248)
at com.sun.java.swing.JComponent.paintImmediately(JComponent.java:2158)
at com.sun.java.swing.DirtyRegionManager.paintDirtyRegions(DirtyRegionManager.java:277)
at com.sun.java.swing.DirtyRegionManager.actionPerformed(DirtyRegionManager.java:208)
at com.sun.java.swing.Timer$DummyComponent.processEvent(Timer.java:256)
at java.awt.Component.dispatchEventImpl(Component.java:1764)
at java.awt.Component.dispatchEvent(Component.java:1704)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:63)