Comment # 1 on bug 1168389 from Ignaz Forster

This message appears since transactional-update 2.21 and its switch to rbind
mounts for /proc, and appears in connection with "autofs" mounts. In /proc we
can find the following submount:

systemd-1 on /proc/sys/fs/binfmt_misc type autofs

Now if `umount -R` is called this will inadverently trigger the *mount* of the
subvolume if it hasn't been mounted before; `umount` however did not see this
coming and fails consequently.

Fabian tried to fix this in https://github.com/karelzak/util-linux/pull/1002,
let's see what upstream says about that patch.

The umount is still successful by the way: transactional-update will try twice,
and the second attempt succeeds as expected. I guess it would be a good idea to
only print a warning if the first umount attempt fails, and only print the
detailed error if the second attempt also fails...