Omniauth-facebook keeps reporting invalid_credentials

Question

I am trying to implement omniauth-facebook as described in Railscast #360 and have run into quite a roadblock. When I click on the signin link, I get the desired popup aski

Answer 1

I had a similar issue where it was working for 1 user but getting the Authenticating error for the 2nd user.

Disabling the Sandbox mode (Apps > Settings > Advanced) seems to have fixed it.