I've seen this happen occasionally, but this time it happened in all sub-forums I follow. After reading new messages and returning to the forum, they still showed as "New" and h had to be marked as "Read" manually.
I only use Firefox for daily browsing, Chrome sometimes for experimenting. I've only seen this occasionally, but the last time it happened on all three forums that had new messages. Not a big deal, just wondered if anyone else has seen something like that.