I have a worksheet that I created in Excel 2003 (XLS format).
If I open it in Excel 2007 and then try to save it, it reports the following error:

This workbook contains data in cells outside of the row and column limit of the selected file format. Data beyond 256 (IV) columns by 65,536 rows will not be saved. Formula references to data in this region will return a #REF! error.
I know my worksheet doesn't have any cells outside this range.
I know that there are no formulas directly referencing cells outside this range.

When I click the "find" link it finds cells that have validations on them (drop down lists). Those lists get their values by doing lookups with named ranges.

Can anyone help?