Using Excel 2010 on machine with 8 GB of memory.
Unusual problem in large spreadsheet today. The large xlsx was linked to a small xls spreadsheet to look up some data (used the vlookup function). In my formula there were numerous references to the current row, including an alternate lookup based on sex (if sex="M", vlookup(), if sex="F", vlookup(), 999)
When I copied the formula down I noticed some #REF errors way down the spreadsheet, and noticed that they began after row 65536. I redid the copy with no improvement, deleted the offending rows and closed the large spreadsheet and reopened and redid the copy still with no improvement. This is the part that was especially strange. Some of the references to that row in the formula were fine (65537) while one of them produced #REF.
Knowing that this looked like some problem related to old versions of Excel, I closed the small xls file, resaved as an xlsx, and then relinked for my search. Now when I copy down the formula there is not problem!!!