Google has fixed an annoying feature that used to mark entire Gmail's threaded conversations as “unread,” after you cliked “mark as unread” button. When you open next time, you’ve to remember which messages you already read and which ones you didn't. Now if you're reading a conversation that had unread messages when you opened it and you mark it unread, Gmail will only mark those messages that were unread when you opened the conversation in the first place.