When clicking "sign in" the user is presented with a page that asks to 1) Create a new account , or 2) Link existing account.
Why are they asked to do anything? The values are already filled in. It should automatically create a new account based on the authenticated URL parameters if it's not in the database already.
This was requested in another thread. The reason for the confirmation form is that, for example, when the admin of the board first sets up SSO for the forum, he will want to link his account to the external site, but he'll already have a username on the forum. What then?
Obviously he needs a form that lets him log into an existing account as proof, and then link the external credentials with that account.