FULL PRODUCT VERSION :
java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) Client VM (build 14.2-b01, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
JLabel is able to render animated GIFs. While most of the time this is working well, with some GIFs there are problems. One example is, when using a transparent animated GIF that is consisting of a combination of differential frames and full frames, JLabel is incorrectly filling the complete drawing area with the background color when drawing a full frame after a differential frame. This leads to the fact, that JLabel flickers at that moment, while the same GIF file is playing flicker-free in other viewers, like Internet Explorer or Firefox. This is not acceptable. While it is clear that JLabel is no first class player, it is obviously a bug and should get fixed.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Send me your email address and I will send you a ZIP file containing a complete project, consisting of a simple Java source and two identical files. One is flickering, the other is not. The difference between them solely is that the flickering one is using a combination of diff frames and full frames, while the smooth one is always using full frames.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Flickers when a full frame is drawn after a diff frame on a transparent background.
ACTUAL -
Flickers when a full frame is drawn after a diff frame in a transparent background.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
There is no error message.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.BorderLayout;

import javax.swing.ImageIcon;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class FlickeringAnimation {
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
JPanel p = new JPanel(new BorderLayout());
p.add(new JLabel(new ImageIcon("OptimizedAnimated.gif")), BorderLayout.NORTH);
p.add(new JLabel(new ImageIcon("FullAnimated.gif")), BorderLayout.SOUTH);
JOptionPane.showMessageDialog(null, p);
}
});
}

}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Do not use diff frames in your animated GIFs.