On a spreadsheet a college has created, which contains links to another
excel file, cells that are pointing to null values in the remote file
are all showing #REF! when the remote file is not available. In my own
tests, keeping Zero Values, Update Remote References, and Save External
Link Values checked, null value cells in the remote file usually show as
zero. So I'm trying to figure out why some of the ones he has created
show #REF!.

As an experiment, I had my college create a new sheet in the same
workbook, with a couple of links to null cells in a file I don't have,
and they show as...zeros just like I'd expect.

I wonder if something could happen like...someone at some time opened
newdata.xls when the linked to file was not available, and when prompted
told excel to update links, and all those cells refs were missing in
action... and were saved that way. Is that a possible scenario? Or, can
an excel file become corrupted?