Situation:

One developer is using Windows XP (64 bit) and provides a copy of his Excel
2007 macro enabled workbook utilizing a substantial amount of VB code to a
second developer who in running Windows 7 (64 bit) and the same version of
Excel 2007 as the first developer.

VB code modifications were made by developer two and the file saved, then
given back to developer one. When developer one opened the modified file,
many of the macros in the workbook ceased to function properly for various
reasons that seemed like they were related to unexplanable, incorrect
workbook references

The exact same code modifications made by developer two and manually entered
into the VB code of the original file residing on developer one's computer
resulted in a completely error free, working program.

Even if the original file is left completely unmodified, if the second
developer clicks to save the file and sends it back to the first developer,
the same issues occur. Multiple file transfer attempts included ftp, e-mail,
and flash drives to ensure it wasn't the method that was causing the problem.
The second developer also attempted to run in a variety of compatability
modes before saving the file to see if that changed the results-- it did not.

Any insight as to what could be causing this would be much appreciated.
Thank you in advance.