Don't know how to communicate this without pictures, so here goes:
On every Forum thread page about 700px down on the right edge of a thread's post content, there appears just the sliver of a banner ad. Using a portion of http://umd.net/forums/supply_and_demand as an example, I've red-circled that sliver in the screen-grab below (with the black background). After testing things out, I believe this can be fixed by adding just one more closing < / div > tag just before the "site_footer/banner" section. Using a screen-grab of the "View Page Source" of that same sample post, I've indicated in red where that extra closing tag would go (line 505 on my Firefox Page Source view, towards the bottom before the "end content area" comment). Once that extra closing tag is added (on my test page), the sliver goes away and the banner ad appears at the bottom of the page.
Obviously this glitch-fix involved a bit more than my one extra < / div > tag, lol. Congrats on figuring all that out. Looks much better all over now! Thx!!