Underhanded contest: The OS war
Python

This program opens the image specified on the command line and displays it.

import Image
import sys

with open(sys.argv[1]) as f:
    im = Image.open(f)
    im.show()


Works on linux, does not work on windows. 

This is due to the way windows opens files. Binary mode must be specified for this to work properly on all operating systems.