What may cause using the "Log in with Twitter" button on the log-in form to work intermittently?
When using (testing) this feature, occasionally I will receive "An unexpected error occurred. Please try again later.", however, when trying again a few seconds later it will work as expected (requesting permission on Twitter).
One more thing, assuming everything works as expected, and you decide not to go ahead with the authorisation, and you click "Cancel", and then "Return to [site]", it will redirect you to an error message: "An unexpected error occurred. Please try again later".