Dealing with manual accounts and Shibboleth

One of our colleges use Shibboleth as main authentication method. So when you click on the login link, you are redirected towards the authentication server.

But we also need to have some manual accounts for special guests. So we added a login block at the very bottom of the front page with lots of "returns" so it is hidden from users. Special guests must scroll down to have access.

It works fine until a user enters the wrong password. Instead of getting an error message, he/she gets redirected to the Shibboleth Auth page.

Is there a way to easily have both auth methods side by side?

François