This is my first post, so let me get a few things out of the way. I currently run support for a valuation and fraud examination company of roughly 70 users. I've been presented with a problem that I have not been able to find a solution for online (probably because I'm not wording it properly). My knowledge of Excel operations and vocabulary is limited, so I apologize in advance for screwing anything up in my description of this problem.

A user contacted me this morning because he had an issue with two spreadsheets he was working on for a client. Spreadsheet A was sent several months ago and had a set of numbers inputted (Numbers A). As it turned out, the client had sent the wrong set of numbers with that spreadsheet, so he emailed a new one to us yesterday (Spreadsheet B) with a new, correct set of numbers (Numbers B).

The user saved Spreadsheet B to a file he could work on off of his C drive (Working File). He was working with Working File open, when he decided to check something from the original, incorrect spreadsheet, Spreadsheet A. As soon as he opened Spreadsheet A, the numbers in Working File which were originally correct (Numbers B), immediately changed to the incorrect numbers (Numbers A) without notification.

Fortunately, he noticed this and opened the correct file, Spreadsheet B, to copy the correct numbers (Numbers B) back into Working File. However, as soon as he opened Spreadsheet B, Working File again changed all the numbers without notice to the correct numbers (Numbers B).

My two questions are:
1. Why is this happening? I imagine there is some sort of hidden temp file that is being overwritten.

2. How do I make sure this doesn't happen in the future?

To us, this is a serious problem. If the user hadn't picked up on the fact that the numbers had changed, the entire valuation could have been incorrect and we not only would have lost the client, but also could have been sued in the process.