That's true -- however, that can occur under normal circumstances anyway, depending on latency. TCP packets can arrive in a different order than they were sent, I believe.
If you wanted to have completely accurate (chronologically) messages, you'd be best off sending a timestamp with each message. But then again, the clients' clocks probably wouldn't be synchronized, so you'd have to use a reference time (say, the server) to do that. Of course, either of these schemes would be open to cracking -- modify the client a little, and you can have a message appear anywhere in the list.
You could fix half of the problem by only sending times from the server to the clients, I suppose.
