Formulae are being saved incorrectly when saving in the Excel formats. The
following simple procedure demonstrates the problem:

The attached document is a spreadsheet of numbers in a 85x85 grid. The numbers
decrease in value by one as one spirals into the centre. Most of the cells
calculate its value by subtracting one from its neighbour. The exception is the
blue coloured lower left diagonal - the values are calculated using a different
formula. I wanted to change this so that every cell is calculated by subtracting
one from its neighbour. Easily done:

1) Remove formula from cell A85 by typing in its direct value (7225)
2) Change cell B84 to calculate its value from its neighbour - use the formula
=A84-1
3) Copy B84 and paste into each cell along the blue diagonal up to and including
the centre (AQ43)
4) Check everything okay by changing B84 to 7224 - the numbers still spiral in
and the centre cell should read 0 instead of 1

Now save the document in any one of the Excel formats, then StarCalc5 format
then OOo format. Close OOo. When the Excel format document is reopened in either
00o or Excel, many of the formulae are corrupted - (the centre cell no longer
contains 0 and it is ultimately dependent on every other cell in the square).
The OOo format document opens correctly, although it takes a long time to open.
The StarCalc5 format also opens without any formulae corruption.

Note that if the saved .sxc version is opened then saved in Excel format, no
corruption occurs, the spreadsheet is saved perfectly.

Also note this: 
Open original .xls file
Save in .sxc format
Close OOo
Open OOo and the saved .xls document
Make changes as described 1-5 above
Save in .xls format
Everything okay!