Please see:
http://cvsweb.netbsd.org/bsdweb.cgi/src/usr.bin/sed/

Now take a peek in the Attic.

Notice there are two Makefiles--one in the normal sed/ and one sitting 
in the Attic. When a new version is added to CVS, doesn't it resurrect 
the file from the Attic and begin adding new revisions to that 
resurrected file?

How might this state come to exist? Did an older version of CVS not 
resurrect files from the Attic?

What is the correct interpretation of these two files? Is there one 
revisions 1.1 which is "authoritative" in this case, and one which 
"should" be ignored, or does the data in the Attic/Makefile,v have any 
further meaning?

These kinds of inconsistencies are all over the place. Do they matter?

Also, there are a pile of branch tags scattered around in the CVS tree 
that don't have any associated RCS revisions. How should that 
information be interpreted? For example:

http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/compat/common/vm_43.c

... the file vm_43.c,v has the tag "netbsd:1.1.1" in the RCS file but 
there are no associated revisions: no 1.1.1.1, no 1.1.1.2, no nothing. 
Is that normal or am I missing something?

Much thanks for your patience.