cvs update not pulling in changes

[David Carson]

Two of my developers have discovered a very strange problem.  Let's
call them Bob and Joe.

- Bob has version 1.2.2.3 of helper.c in his working copy.  He
modifies it and is preparing to check it in.
- Joe also has 1.2.2.3, modifies and commits before Bob has a chance.
- Bob does a 'cvs update' (using WinCvs), which tells him that he has
a modified copy of helper.c.  However:
 . it does not pull in the _changes_ from Joe
 . it _does_ update the version to 1.2.2.4
If Bob commits now, he undoes Joe's changes and adds his own changes.

This behavior only happens on this one file.  I tried to recreate by
doing my own change to helper.c.  I was unable to recreate it.  I
don't know if the difference was the fact that I am involved, the fact
that we are now one revision further along, or what.  I'm mystified. 
It has happened to Bob twice.

Unfortunately, I have not been able to look at the file in Bob's
working copy _before_ he does the offending update.  If I get that
chance, can anyone think of something I can look for to indicate why
the update is not pulling in Joe's changes during the update?

Server: 1.11.16
Client: WinCvs 1.3.8.1 (Bob's)
        WinCvs 1.3.13.1 (Joe's)