Sticking with the theme of the health of your spreadsheet, have you ever notice spreadsheet become inexplicably bigger? Suddenly becoming 20MB instead of 2MB.
One cause is unused rows and columns.
How do I check and how do I fix it?
Simple, on each worksheet in your file press End then Home.
Your cursor will jump to the bottom right active cell. If this happens to be in column XFD or similarly far across your spreadsheet, or down in row 1 million and something then this may be your issue.
To fix it you need to delete all of the columns / rows between your data and this rogue cell and then Save (and sometimes re-open) your file. For complex files you may have to resort to clearing out smaller sections each time as the calculation load otherwise causes an “Insufficient Resources” type warning to appear.