It occurs because there is a limit to the number of different formats
that Excel can keep track of.
You can correct it by reducing the number of different formats you
have. For example, click on the intersection of the row and column
identifiers to highlight the whole sheet (or CTRL-A twice for XL2002
and later) then set the font and font size for the whole sheet in one
operation, rather than individually for each cell. While you have the
cells highlighted, you can set the background colour and foreground
colour for the whole sheet, turn off any borders, emboldening,
undelining etc etc. by making use of Format | Cells.
Hope this helps,
Pete
