This is related to Group IM sessions not starting with big groups. When you start a new Group IM session - for example by double clicking on the group entry in your Contacts / Groups list - you will often see a line like this: [11:34] Starting session with Second Life Mentor please wait. While this line sits there, some messages might already come in and fill up the chat window. Once the viewer gets the acknowledgment from the server that the IM has started, the line gets removed, but at the same time, all remaining chat under the line gets deleted, too. Probably the viewer doesn't delete the line itself but everything from the line to the end, assuming that no chat could have happened while waiting for the session to be acknowledged by the server.