With JTextPane the last line (last paragraph) is not showing. This bug occur with follow conditions:
 - The conten type must be change before.

 - The conten type must be text/plain

 - The caret position must be set to 0.
workarounds:

 - create a new JTextPane if the conten type will be changed

 - does not set the caret to position 0

 - use JEditorPane instead of JTextPane

See the follow test program to reprodce the problem. We have reproduce it with Java 5 and Java 6.

import javax.swing.*;
public class TestShowTextWithoutLastLine {
    public static void main( String[] args ){
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation( WindowConstants.DISPOSE_ON_CLOSE );

        JTextPane text = new JTextPane();
        frame.add( text );
        String anyText = "first line\nsecond line";
        text.setContentType( "text/html" );
        text.setText( anyText );
        text.setContentType( "text/plain" );
        text.setText( anyText );
        text.setCaretPosition( 0 );

        frame.pack();
        frame.show();
    }
}
Further concern from the submitter and still think it is a bug:

The evaluation field show a solution but the API documentation say that the method setText() is
thread safe.
The follow modified test program show that the problem occur inside of setText()
if it is a thread problem.
Only the method setText() is not call from the event thread and the
problem repeat. The API doc says that
this method is thread safe.

import javax.swing.*;
public class TestShowTextWithoutLastLine {

    public static void main( String[] args ) throws Exception, Exception{
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation( WindowConstants.DISPOSE_ON_CLOSE );
     
        final JTextPane text = new JTextPane();
      
        frame.add( text );
        frame.setSize(300,200);
        frame.show();
      
        final String anyText = "first line\nsecond line";
        SwingUtilities.invokeAndWait( new Runnable() {
            public void run() {
                text.setContentType( "text/html" );
            }
        });
        text.setText( anyText );
        SwingUtilities.invokeAndWait( new Runnable() {
            public void run() {
                text.setContentType( "text/plain" );
                text.setText( anyText );
                text.setCaretPosition( 0 );
            }
        });
    }
}