The unshare_one() in libzfs could fail with EZFS_SHARENFSFAILED at line 834
here:
831    /* make sure libshare initialized */
832    if ((err = zfs_init_libshare(hdl, SA_INIT_SHARE_API)) != SA_OK) {
833        free(mntpt);    /* don't need the copy anymore */
834        return (zfs_error_fmt(hdl, EZFS_SHARENFSFAILED,
835            dgettext(TEXT_DOMAIN, "cannot unshare '%s': %s"),
836            name, _sa_errorstr(err)));
837    }
The correct error should be EZFS_UNSHARENFSFAILED instead.