I update files monthly and save them each month with a new name (eg. Jan, Feb, etc.).

In the file I have a reference from one tab to another tab within the same file. So every time I re-save this file with a new name, it should still reference the cell in the other tab - same file.

BUT on 2 occasions this reference changed to an "external" reference to the previous month's file.

I've been doing this every month for over 2 years and this problem has only happened 2 times. I know others that have experienced this, but nobody knows what caused it.

Does anyone know what would cause this? The biggest problem is that I may not notice this has happened for a long time.

One other thing I might add. I use an older version of Excel (2000). I share these files with other people who may have newer versions (I'm not sure who has what). Can sharing these files with others over a network system have something to do with this? Can opening "read only" when someone else is in the file and then saving as a copy do this?