Simple Q re Extraneous filter-flag

The following recipe gives me "Extraneous filter-flag ignored":

    # Some stupid Microsoft email program adds unreadable uuencoded junk.
    :0fhw
    *       ^X-Ms-Attachment: WINMAIL\.DAT
    {
            :0fbw
                    |sed '/^begin [0-7][0-7][0-7] WINMAIL\.DAT$/,$d'
    }

procmailrc(5) says

          On a nesting block,
          the flags  `H'  and  `B'  only  affect  the  conditions
          leading  up to the block, the flags `h' and `b' have no
          effect whatsoever.

I can't quite decipher the language of the man page.  What is procmail
complaining about?  The outer 'h'?  The inner 'b'?  Or the inner 'f'?

Thanks,
== Bob