I've seen this happening many times, not only under Panther, but also under Jaguar and on several machines like G4 (Dual MD 1 GHz and 1.25 Ghz) and G5 (1.6, 1.8 and Dual 2 Ghz). No matter what disc, rebooted app, rebooted machine, repaired disc permissions, it just happens at random.
I have been searching a couple of times for it on the web and it seems a very occasional problem though I've seen it happen may many times.