I quote from K.A.T. in another thread:
I thought I'd noticed this happening (new messages appearing with no
new icon) myself but I wasn't sure...
Has anyone else experienced this?
Haven't particularly noticed that because I tend to use "Show unread posts since last visit" - But I have noticed that this isn't always accurate.
Weird. I've never seen the new indicators ever screwed up in an SMF (or YaBBSE) board (or ever worked correctly in other boards!) till now. The spell is broken?
Does the forum store the new/read post data in one's profile or in cookies? The latter has more potential to cause trouble, especially with IE.
SMF (this forum) tracks read topics from the server, on a per user basis. The mechanism can be tricky if you login but do not read all of your unfinished topics. This behavior is not perfect, but is a reasonable compromise when dealing with a large number of topics and users.
Thanks for the clarification.