I've noticed that the main forum list does not correctly display the latest post in a forum under certain conditions:
Primarily, I notice this when the forum has a subforum, a new posting was made in the main forum and I showed it as new. I have since displayed this posting, but in the meantime a posting was made in the subforum. When I go back to the forum lists, it shows that there is a new posting (as expected) but the new posting displays as the one I have already read in the main forum. No amount of viewing any of the postings in the main forum will clear the new posting icon on the forum list. The only way to clear that is by viewing the posting in the subforum.
Most of this behavior is really correct. The bug is that sometimes it displays the posting from the subforum as the latest post and other times it displays an older posting from the main forum. I see this most often with less used forums like the Composer's Lounge with the Chopin subforum.
It's more an annoyance, because I keep going back to a topic in the Composer's Lounge thinking that there is a new post on a topic I'm following there.