Data Validation in XL2007 suddenly stops functioning

[ker_01]

I created a workbook in XL2003 that included some named ranges on Sheet2 that
fed some data validation lists on Sheet1. Everything worked as expected. My
colleague opened up the workbook in XL2007 and used it several times, and
everything worked as expected.

Then she called me, saying that it was broken. Of the 6 named ranges and
related data validation columns on the other sheet, none of them were working
anymore. I went over to her PC, and checked everything I could think of- I
could set up working data validation test cells on Sheet2, but could not get
the ones on Sheet1 to work. Data validation was still set up properly on
those cells, and the named ranges still existed and pointed to the
appropriate sources. The sheet was not protected, and I could find nothing
else unusual.

I finally created a new sheet (Sheet3) and copied the entire sheet (Sheet1)
and pasted it over. The data validation all worked as expected on Sheet3 with
no further intervention, but still does not work on Sheet1.

While we effectively have a fix (we just deleted Sheet1 and renamed Sheet3),
I'd like to understand what might cause a single sheet to suddenly not work
with data validation. The cells were selectable and we could type in whatever
we wanted, it just never showed the drop-down arrow and the related list(s).

Any ideas what would cause this type of issue?

Thank you,
Keith