The "render_403" method isn't available from the API controller, only the UI controller. Now the smart proxy auth is called from the API reports controller, when smart proxy auth fails it calls this method and hits this error:

Started POST "/api/reports" for 127.0.0.1 at 2013-09-24 10:22:45 +0200
Processing by Api::V2::ReportsController#create as JSON
  Parameters: {"report"=>"[FILTERED]"}
Verifying request from ["localhost", "localhost.localdomain", "localhost4", "localhost4.localdomain4"] against []
No smart proxy server found on ["localhost", "localhost.localdomain", "localhost4", "localhost4.localdomain4"] and is not in trusted_puppetmaster_hosts
undefined local variable or method `render_403' for #<Api::V2::ReportsController:0x007fcbd80ed5c0> (NameError)
/home/dcleal/code/foreman/foreman/app/controllers/concerns/foreman/controller/smart_proxy_auth.rb:28:in `require_puppetmaster_or_login'

(regression)