Sample error log in /var/log/messages:
Oct 28 09:15:01 gentoo-host crond[16678]: PAM adding faulty module: /lib64/security/pam_unix.so
Oct 28 09:15:01 gentoo-host crond[16677]: (user) PAM ERROR (Module is unknown)
Oct 28 09:15:01 gentoo-host crond[16677]: (user) FAILED to authorize user with PAM (Module is unknown)
Oct 28 09:15:01 gentoo-host crond[16678]: (user) PAM ERROR (Module is unknown)
Oct 28 09:15:01 gentoo-host crond[16678]: (user) FAILED to authorize user with PAM (Module is unknown)
