When you try to save your workbook, you may get this warning:
The following features in this workbook are not supported by earlier versions of Excel. These features may be lost or degraded when you save this workbook in an earlier file format.
Significant loss of functionality
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.
This warning is due to an Excel problem that has been reported to Microsoft. Just click Continue to proceed with saving the workbook. Despite the warning message, you will be able to open the workbook later with no loss of data.