I've always been a bit suspicious, but if I remember it correctly then I've been told that the "mark all read" will never hide messages which you haven't seen. I got a little chat with rcook on a thread some time ago, when I asked exactly this question: What happens if I start to read multiple pages and then hit "mark all read" on (let's say) page 5 - I think rcook told me that it will not hide new massages/replies that were posted after I started reading page 1. This sounded reasonable, because often after hitting "mark all read" the "message counter" immediately showed non-zero. But... yesterday I started reading 26 messages (page one said 1-25 of 26), then I went to page two - and it suddenly showed two remaining posts (labelled 26-27 of 27). I hit "mark all read"... and the message counter showed zero! I moved in, and there was nothing indeed, but when I went to "archive", a new topic was on front. Is this a change in the way how "mark all read" works, or just an unlucky glitch?