I have 2 separate Excel documents and I want to join it together so:

-Document 1: simple document. I insert 3 columns (A,B,C)and some numbers on each colums.

-Document 2: simple document. I insert 3 columns (A,B,C)and some numbers on only 2 of them.

1.Now I want do have in column 3 of "document 2" (that has no numbers) the same number as column 3 in "document 1".

2.I do drag and drop and all is ok.(It creates a nice formula)

3.After saveing the files I decided to add a column and some numbers between column B and C in "document 1". ( now I have 4 columns and not 3)(A,B,C,D)

4.Now I go to document 2 and the value is not the one I expected to have (first value of column C that now is column D)

Why is this happening? Is it some unknown bug? Do I need to update the links between the 2 files all the time?

This only happens if you change the file while the other one is closed. In order to maintain relationships, you need to have all files open that refer to the one your are changing.