I've been working on an Excel file with no problems. Then yesterday I opened it and got the error in the subject line. Certain - but not all - of the formulas in the workbook now simply contain "=#N/A". I fixed the formulas and re-saved it. I opened the file again today and got the same problem. Older versions of the file with the same formulas (maybe slightly different) are opening fine. There are no macros in this workbook. I'm using Excel 2007. Any ideas why this might be happening and how to stop it?