Following exception can occur if document that doesn't conform HTML3.2 spec is displayed in JEditorPane:
Thu Jul 12 16:16:47 CEST 2001: java.util.EmptyStackException: null
java.util.EmptyStackException
        at java.util.Stack.peek(Stack.java:82)
        at javax.swing.text.DefaultStyledDocument$ElementBuffer.insertElement(DefaultStyledDocument.java:1623)
        at javax.swing.text.DefaultStyledDocument$ElementBuffer.insertUpdate(DefaultStyledDocument.java:1361)
        at javax.swing.text.DefaultStyledDocument$ElementBuffer.insert(DefaultStyledDocument.java:1231)
        at javax.swing.text.DefaultStyledDocument.insert(DefaultStyledDocument.java:194)
        at javax.swing.text.html.HTMLDocument.insert(HTMLDocument.java:212)
        at javax.swing.text.html.HTMLDocument$HTMLReader.flushBuffer(HTMLDocument.java:3014)
        at javax.swing.text.html.HTMLDocument$HTMLReader.flush(HTMLDocument.java:1921)
        at javax.swing.text.html.HTMLEditorKit.read(HTMLEditorKit.java:240)
        at javax.swing.JEditorPane.read(JEditorPane.java:421)
        at javax.swing.JEditorPane.read(JEditorPane.java:439)
[catch] at javax.swing.JEditorPane$PageLoader.run(JEditorPane.java:468)

This can be observed for example when NetBeans IDE is running. Set HTML browser to Swing HTML browser in setup wizard and try to open web browser - homepage http://www.netbeans.org should be opened but the result is this exception. Also see http://www.netbeans.org/issues/show_bug.cgi?id=13062