We just noticed this on http://bugs.darcs.net/issue1751 (roundup 1.4.4
via Debian Lenny).

Digging around a bit, I noticed this roundupdb.py:

            if not inreplyto:
                # Default the reply to the first message
                msgs = self.get(nodeid, 'messages')
                # Assume messages are sorted by increasing message
number here
                if msgs[0] != nodeid:
                    inreplyto = messages.get(msgs[0], 'messageid')
                    if inreplyto:
                        writer.addheader('In-Reply-To', inreplyto)

Could that be the culprit?  It seems like the very first message in an
issue gets this, whereas subsequent messages have correct-looking
in-reply-to headers.

Is it actually bad for messages to be replies to themselves?  Why not
just not have an In-Reply-To header for just cases?