Impossibly complex problem

I have a really irritating complex problem, and I doubt anyone can
solve it, but I'll post anyway -- just in case.

Inbound mail from this procmail list is getting munged such that the
string "Subject:" is getting replaced with a space.  But this doesn't
happen all the time, just most of the time.  It seems the anomaly does
not occur when I have exactly one procmail message at the time I
retrieve email, and no other messages.  As soon as I download multiple
messages, all the procmail list messages lose the subject header
field.

At the same time this problem began, messages from the wget mailing
list also began losing the "subject:" header, but are then dumped in a
folder called "Segmentation," instead of wget_2005 (procmail messages
are correctly dumped in procmail_2005).  I never defined a folder to
be called segmentation, so procmail is seriously screwing things up.

There is no reason for procmail to have this problem in isolation,
because the sole script that processes my procmail mail is generalized
and processes other mailing lists (which don't have this problem):
  
  BEENTHERE_PREFIX_ML = (\
  dia-list\
  |gnupg-[a-z0-9]+\
  |latex2rtf[a-z0-9.-]*\
  |magick-users\
  |sox-users\
  |gimp-user\
  |procmail)
  
  BEENTHERE_SUFFIX_ML = (\
  (lists|informatik)\.RWTH-Aachen\.DE\
  |[a-z0-9.]*berkeley\.edu\
  |[a-z0-9.]*sourceforge\.net\
  |gnome\.org\
  |gnupg\.org\
  |imagemagick\.org)
  
  # BeenThere mailing lists
  #
  :0
  *$ ^X-BeenThere:(_dot_)*$BEENTHERE_PREFIX_ML(_at_)$BEENTHERE_SUFFIX_ML
  *$ ^X-BeenThere:.*\/$BEENTHERE_PREFIX_ML@
  * MATCH ?? ()\/[^(_at_)]+
  {
    LISTNAME=$MATCH
  
    # commented out code here removed for clarity

    :0 :
    mailing_lists/${LISTNAME}_${YEAR:-0000}
  }  

And wget is another class of lists that is not a "Beenthere" list, and
has its problem in isolation from the other lists of its class.

My library of scripts is very large.  This problem is impossible to
troubleshoot, because when I process one message at a time and watch
the log files, all goes well.  As soon as a batch of messages go to
procmail, messages from the procmail list and wget list are hosed.
The logs are impossible to follow because the output is all out of
sequence.  To keep things thoroughly confusing, I'm running two
processors with the symmetrical multiprocessor kernel.

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