If an exception goes uncaught out of LDAPBacked.authenticate() function, the caller side (django.contrib.auth.authenticate()) doesn't catch it  and thus stops processing of authenticating backends chain. So e.g. a locally created 'admin' user isn't able to login at all (the LDAP search returns nothing so accessing of search[0] generates IndexError and voila -- after switching to LDAP-based authentication admin can't login and can't grant a LDAP user administrative permissions)