This is a Microsoft-unknown bug, which I, and a few of my ex-colleagues have
experienced recently. Happens when you open an Excel 2003 workbook  in 2007,
and then save  it as any other name, whether it be 2003 of 2007 format.

What triggers the bug exactly I don't know yet. Filesize greater than x?
Workbook contains internal and external links?

If you can explore, and give feedback, that would be great.

Many companies use old 2003 models, change data, and make important
financial decisions based on the "changed" results.