Comment 19 for bug 160413

Another way to double-check that is to edit /etc/init.d/networking, look for:
stop)
        if sed -n 's/^[^ ]* \([^ ]*\) \([^ ]*\) .*$/\2/p' /proc/mounts |
                grep -qE '^(nfs[1234]?|smbfs|ncp|ncpfs|coda|cifs)$'; then
            log_warning_msg "not deconfiguring network interfaces: network shares still mounted."
            exit 0
        fi

        log_action_begin_msg "Deconfiguring network interfaces"
        if ifdown -a --exclude=lo; then
            log_action_end_msg $?
        else
            log_action_end_msg $?
        fi
        ;;

And change:
if ifdown -a --exclude=lo; then
by:
if true; then
(or anything else that prevents ifdown from running)

Then do a normal shutdown. This way to test is safer than the other one, too :-)