I did some investigating of the problem . . .
The page I linked to had this URLhttp://www.homebrewersassociation.org/forum/index.php?topic=11091.msg137880#new
This is the "APAs...AIPAs...Comp Yeast Strains?" thread, correctly identified as topic=11091
However msg137880 doesn't exist on the page.
I've got: 137866 as the first message, followed by 137870, 137874, 137875, 137879, 137891, 137892, 137896, 137900
It can't go to message 137880 because it is not on the page, so it reverts to the top post.
Based on my recollection of the thread, the last post I read was 137879 - could it be as simple as it marking the "new post" in the url as the last post read +1? This isn't correct, but if there was some logic that said "if msg# does not exist, go to the first message after msg#" then it would take you to the first new post. And if it was expecting this logic but it got broken somehow, that would explain what we are seeing.
I could be completely wrong about how this works on the back end of course, I'm just making guesses. But the url it is trying to link to doesn't exist.