I ran into a Leopard system the other day that was behaving erratically. I looked at system.log, and encountered some errors I'd never run into before.
Lots of those, and for various UIDs and myriad processes. I thought I'd open up dscl and see what was up with the system users.
Apparently, this system had all of 3 local users. This is a bit strange, as any normal Leopard system should have at least 40.
What had happened, for reasons that I haven't been able to determine, was the .plists for most of the system users and groups (located in /var/db/dslocal/nodes/Default/) were missing. I put the machine in target mode, grabbed those from a functional Leopard system, and everything was fine.