I'm seeing some strange behavior with the nwchem package. If you hack the package to add:

url = "http://www.nwchem-sw.org/images/Nwchem-6.6.revision27746-src.2015-10-20.tar.gz"

and run:

$ spack versions nwchem
==> Error: Couldn't detect version in: http://www.nwchem-sw.org/images/Notdir_fc.patch.gz

This URL comes from urls_for_patches. Any idea why it would be picked up instead of the package-level url?