This odd fault occurs just occasionally when, end of month, i have about six small spreadsheets open.
To update them i have to include a new month filename. So i do a 'save as' to the new filename, make all the changes required and finally 'save' before emailing the new sheets out. Then i get an email back from the recipient saying i've emailed the previous month sheet (although the filename shows the current month). So i open the returned sheet - and yes it's the wrong sheet. I close that sheet and then open it again from my computer copy with exactly the same filename - to find that it now has the correct sheet. I've changed nothing but somehow Excel has associated the wrong sheet to the filename. The more separate file named sheets i have open, the more likely this fault seems to occur. ??