gistfile1.rb

@invalid_user = Factory.create(:user)        
@invalid_user.stub(:valid?).and_return(false)

User.stub(:new).and_return(@invaild_user)
        
post :create, :user_name  => "some user name that won't even be used"

https://gist.github.com/723630/5906238dfb0c973a89fc15727eab0dddfe40a21a#LID4

I did miss it completely: you will not be able to pass a whole ruby object through the post parameters I think.
My guess is that somewhere in the way the mock request is processed that thing will be smashed.