OS X Lion 10.7.4. I did work on a Microsoft Excel 2011 spreadsheet (saved in .xls format) and noticed a file on my desktop that said it could not be trashed. It disappeared when I re-started the Finder. But the next time I opened the Excel spreadsheet, the work I had done had been lost — except for a 'sum' field which displays the total from the lost fields but says it contains the function '=SUM(#REF!)'. Perhaps the Excel file on my desktop was a temporary storage file for the new data I added to the spreadsheet. Getting rid of this file may have trashed the data it may have contained. I've just done work on another spreadsheet and a new 'non-trashable' Excel file has appeared on my desktop. I hesitate to re-start the Finder in case I lose the work I have done. Has anyone else experience the same problem, and is there a solution?
There is clearly a bug in Excel 2011 for files on the Desktop where Excel does not clean its scratch files during editing and quitting. Like you, only rebooting seems to get rid of them. The only workaround I have sorted out is to simply not edit Excel docs on the Desktop. I have not seen any data loss issues related to those files on the Desktop.