Here is a Microsoft Knowledge Base article about that:
http://support.microsoft.com/default...b;en-us;213904

This happens in large spreadsheets that are format-intensive. Excel
can keep track of about 4000 different combinations of formats; beyond
that you get the error message. I have corrected this in large
workbooks by deleting unused sheets and by going to infrequently used
(but mission critical) tabs within the sheet and setting them to a
neutral format: no font color, no background color, no
left/center/right alignment, no patterns, no boxes.