Several of my accounting staff have reported a strange problem. The other day two of them were working on Excel spreadsheets and when they updated an amount in one cell, the formula that added up a range of cells, including the one they changed, did not update. I couldn't duplicate the problem but they said they checked the formula, made sure they clicked off the updated cell, even saved the spreadsheet, etc. These are experienced Excel users so I trust they know what they are doing.
To further complicate things both users were working on different spreadsheets under different versions of Excel (one on Office 2003, one on 2007).
The only common thing between them is that both spreadsheets were opened from our shared network drive. I thought it might be related to loss of network connection, but both said they were able to save the spreadsheets back out to the network. The following day when I looked at the spreadsheets, the formulas were working fine and updating as values changed.