I also got a similar problem when I had too little place left on the boot drive. I think it's an error in the way the finder calculates free disc space.
The finder reported about 200 Mb free, but when I tried to copy some files in the terminal I got a "Disc full" message. So when applications are about to quit and try to save their preferences , nothing gets saved. After I freed up some space on the boot drive, this never happened again. From what I can remember now you should have more free space on the disc than you have RAM.
G4 500 MP 1024 MB RAM
LaCie Electron 22" Display
OS X 10.3.9
iPod + iPod Mini
Sony PC100E FireWire Cam
Static IP via a FreeBSD server/gateway
a lot of other Mac's running Macos 7.5 to 9.1 and NetBSD.
A Few Wintel's on the side (I only use them when I get paid by the hour)