Update:
This bug was seen on Windows NT 4.0 SP6

Update (see evaluation):
This seems to be a problem with animated gifs in general.

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Up until 1.4.0, one could use the following to show an animated gif on a JButton or JLabel:

JLabel animLabel = new JLabel(new ImageIcon("anim.gif"));

This no longer works as of 1.4.0. It seems that the image will animate one or two cycles and then stop.

Try the attached test case, which shows a JButton and JLabel, both containing an ImageIcon with an animated gif. In 1.3.0 and 1.3.1, the gif animates indefinitely. In 1.4.0 it stops after one cycle or so.

(I've attached a sample image "anim.gif" for testing)