When a Shibboleth Identity Provider does not provide a required attribute, Mahara presents a page that says "Site Unavailable" and "A nonrecoverable error occured. This probably means you have encountered a bug in the system." You also will get stuff in the error_log like what's in the file attached.
By comparison, when this situation arises with Moodle, the page tells the user something like this:
"You seem to be Shibboleth authenticated but Moodle didn't receive any user attributes. Please check that your Identity Provider releases the necessary attributes ('REMOTE_USER', 'givenName', 'sn' and 'mail') to the Service Provider Moodle is running on or inform the webmaster of this server."
Especially if people intend to run federated authentication, SAML should fail gracefully if an Identity Provider doesn't provide all the attributes Mahara requires. If nothing else, it at least makes it clear that the problem is probably with the Identity Provider and not one or more bugs in Mahara.
I always forget to include version information. This is running the 1.3_STABLE branch with the fixes for bugs #670546 and #688395.