I have a server with Zimbra Release 8.0.2_GA_5569.RHEL6_64_20121210115059 CentOS6_64 FOSS edition. I use advanced web client and I have the reading pane off so the messages are opened in a new tab within Zimbra. After opening some messages, the new messages opened aren't marked as read. This issue occur with FireFox 19.0.2, Internet Explorer 8 and Chrome. If I refresh the session, the issue is solved.

Best regards.