Recently I've been having a sporadic problem, and I'm not sure how to repro:
Sometimes I'll click on a brand new thread, which I've never seen before, and all of the posts therein will be "collapsed" and gray (i.e., marked as already read). And sometimes this will happen when I look in my profile for threads with new content. If I click in using the Last Reply link all of the posts will be gray, as though I've read them -- which my profile has just said that I have not.
Sorry for the vague description. This has happened just enough times for me to notice and get annoyed by it. In any case I'm pretty sure there's a bug somewhere.