I am not sure what causes the "corrupt" font cache, but I have a theory. :wink:
It seems to happen when a font is activated twice in your font manager or when a font is active in one of the system font folders and then you activate a second in your font manager.
Fortunately, the solution is relatively simple. Download the Font Cache Cleaner script
here.
This will delete the font caches in yoour system, and reboot. All should be OK after that.
You may want to visit Apple's Discussion forum and check out the Mac OS X 10.3 (Panther) User Contributed FAQ section for some more information regarding fonts an font management in Panther. Also, Suitcase X1 (apparently) fixes quite a few issues.
pk