workbook changed by other user since last save...!

I have a workbook on our local network which several people can access - although note that this is not a shared workbook - it can only be accessed by one user at a time.

Very rarely, when I make changes to the workbook and then click save, I get the following message:

"The file <INSERT NAME HERE>.xls may have been changed by another user since you last saved it. In that case, either save a copy or overwrite changes".

Please can anyone tell me why I get this message, and how do I prevent this happening? My guess is that we both tried to access the file at exactly the same time, therefore neither of us got the "workbook locked by user" message.

Thanks.

Just to verify...you have verified that Tools/Share Workbook is not checked?