If the forum thread pages have the "next" and "previous" thread links toward
the bottom of the page you will need to check if those are being mirrored and
filter them out if they are.

+*showthread.php?t* +*=384721* +&page=* - might be too inspecific rules to use
to just get certain threads, especially the +*showthread.php?t* and +&page=*
parts. Those two parts of the rules might grap anything in the thread pages if
other links you don't want contain them.

You need to hover your mouse pointer over links in the forum thread you test
mirrored and look at those links that were saved locally that shouldn't have
been, and then look at those links on the "real" forum thread on the Internet
to see how those links are structured and then revise your httrack rules so
they don't get mirrored.

If this forum has the hybrid and threaded views options and you don't need
those views, if they are being mirrored you can save a little time by
filtering them out, same goes for print view.