This thread popped up on my list of threads with a new post. It shows having a post today. I click on it and see the last post was actually in July. I get a lot of these showing up as having new threads but when clicked only have old posts.
IIRC this happens to threads with polls on the top.
If someone votes on the poll, I shows up as a new post, even if there is no new post.
I admit it can be annoying to people (including me) that use the "Unread posts" link.