I created a stripped-down version of the repo which still illustrates the warnings quite well. Also, it looks like at least some of the warnings may be stemming from the use of FactoryGirl.attributes_for, but only in cases where a hash is passed in with the factory name (have a look at spec/helpers/factory_girl_helpers.rb). Adding some debugging indicated that there is other code responsible, as well, but I didn't really want to take the time to track them all down.

The repo is here:
https://bitbucket.org/ToadJamb/example-factory_girl_instance_var_warning