My list has used subject tags since it was running on Majordomo.

With 14.5, I saw Misc-Options= SUBJECTHDR_SEQUENCE as a useful addition,
because some participants' spam filters start rejecting messages when the
same subject repeats too often. A long discussion can produce a lot of mail,
all with subject "Re: [us] yadda yadda"; I thought maybe adding sequence
numbers would defeat that pattern match.

Early cases looked good. Replies to sequence number 4 correctly recognized
the [us - 4] tag and replaced it, so they came through with subjects "Re:
[us -5]..." and "Re: [us - 6]..." instead. However, today we got this
subject line sequence:
   [us -20] yadda
   Re: [us - 22] [us - 20] yadda
   [us -23] Fw: [us -22] [ us -20] yadda

For fear of subjects that looked like archive catalogs, I removed the
sequence option and announce the change to the list as a reply--producing
  Re: [us] [us -23] Fw: [us -22] [ us -20] yadda

I can see two possible explanations:

(1) We hit an unexpected bug in this feature which causes it to agglutinate
sequenced subject tags after some sequence number (presumably the limit in
testing) is exceeded.

(2) The Previously Unmentioned Extra Factor: I added Misc-Options=
IETFHDR_SUBJECT_TAG between the time the behavior was correct (with new
sequenced tags replacing old ones) and the first observation of accumulating
tags. Is it possible these two options do not work together correctly? or
(to avoid value judgments) do not work together in the most desirable
fashion?

I should mention that one of the participants in the above sequence (the one
sending sequence number 22) does not use IETF headers.

Hal Keen