Tested for cvs-1.11-3 on Fisher (not sure if I reported against the right
version above).

If you do:

        cvs admin -n<new tag>:<old tag>
        
from the top of a workspace, it will abort with the following error if 
<old tag> is not found in a file:

RCS file: /ash/cvsroot/tgx/3rdparty/belcarra/exim/tgx.postinstall.sh,v
cvs [server aborted]: tag `exim' does not exist

It should, instead, make no modifications to that file and continue on 
with the rest of the files.

I will attach a script that reproduces the problem.