Eggs installed from a user's account will want to unpack into the .egg_cache in the user's home directory, even if the eggs are installed to a system location. If this directory is unavailable (for example, if the permissions on the directory have changed or the account has been removed), then a very confusing exception is thrown.
This is a terrible default behavior. Ideally, PYTHON_EGG_CACHE shouldn't exist. Certainly I shouldn't have to export it when I'm running "sudo python setup.py install -f".