Not strictly Linux - in this case it's an AIX system, but...

Has anyone ever seen a situation where a person can't chmod their own files?  The only thing I 
could find that made any sense was that at least some of the files showed ownership by a group 
=number=, not name.  I'm fairly certain that a whole bunch of stuff got migrated over from a SCO 
system via tape archive, and the number in question is typically the group that all the users are a 
member of on a SCO box. Anything else I'm not thinking about here?