I'm not sure what causes it, but the forums often incorrectly show new posts, leaving some out. I have to select a time range instead of whatever the time of the previous visit was to make the posts actually show up, and even then it doesn't list threads as having new posts, even if I haven't read them. This bug makes it harder to make sure I haven't missed anything.
I don't know if it has anything to do with me checking the forums on two different computers and browsers, but it even happens when I continually check the forums on the same computer with the same browser.