Seems I was wrong, but close.

Got text below from http://blogs.msdn.com/excel/archive/...26/474258.aspx, where they are comparing limits in Excel 2003 to limits in Excel 2007.

Probably you have hit the limit on nesting of formulas ...

The maximum length of formulas (in characters)
Old Limit: 1k characters
New Limit: 8k characters

The number of levels of nesting that Excel allows in formulas
Old Limit: 7
New Limit: 64