The new notification callback mode will allow a Client to call the token endpoint with the backchannel grant type.
If the Client calls the endpoint before receiving a notification should the error be the same as for polling, i.e.
The authorization request is still pending as the end-user hasn't yet been authenticated.
Or should there be a different error message.
At the heart of this issue is the question - can a Client configured to receive a notification that auth has happened also poll the token endpoint?