Why is spec calling out to AWS for latest/meta-data/iam/security-credentials/

This is a failure that appears when running specs on one machine but it doesn't appear on others (or on CI server):

1) ManipulateImage#run makes a call out to image service
     Failure/Error: convert_image.run
     WebMock::NetConnectNotAllowedError:
       Real HTTP connections are disabled. Unregistered request: GET http://ift.tt/1h7XB27 with headers {'Accept'=>'*/*', 'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3', 'User-Agent'=>'Ruby'}

       You can stub this request with the following snippet:

       stub_request(:get, "http://ift.tt/1h7XB27").
         with(:headers => {'Accept'=>'*/*', 'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3', 'User-Agent'=>'Ruby'}).
         to_return(:status => 200, :body => "", :headers => {})

Any ideas what I might not have set up correctly on the failing machine?