It's a relatively new computer and came with OSX 10.6.7.
I'm using Numbers 09, ver 2.0.5.
This is a spreadsheet that I created in Numbers. It has eight sheets that are interconnected. The formulas are not complex. I've used some conditional formatting in the header rows. I'm also using footer rows. There are simple graph charts (not 3D) based on spreadsheet data, but no other graphics. I did not use any links or shapes or other graphics. I would call it a simple multi-page spreadsheet with simple graphs.
At the moment it is not a large spreadsheet. Each sheet has no more than 15 rows and columns.
I can't find anything that would cause a save failure after reorganizing. It's puzzling and very annoying.