running same script on same data on two different machines --> different result

[Christopher Brewster]

I am running the same script on the same data on two different
machines (the folder is synchronised with Dropbox).
I get two different results. All the script does is count words in
different files and perform a simple set operation on the word lists.
The laptop is a Macbook Pro (2 1/2 years old) running OS X 10.5.5 with
Python 2.5.1
The desktop is an iMac (brand new) running OS X 10.5.5 also with
Python 2.5.1

I have tried running the script on an ubuntu server with Python 2.5.2
and the results corresponded with my laptop's output.
How can I find out the cause of this anomaly? What tests can I
perform?

Thank you,

Christopher Brewster
Aston University