Today I received a different document with invalid references & that one
showed the "Error..." so it appears there is something messed up with the
references that weren't updating in the first document.

After playing around with the document a bit, I found that for some reason
those particular cross reference fields were locked, which prevented the
Error from appearing.  

Sorry ... I never thought to check that earlier since we generally don't
lock our references. Our end-user did that on his own.

Thanks for your help.  I'm very glad to see that these errors will still
appear.