It's possible this behavior is evident in other areas, but is right now most noticeable when sending a message and using the case search. After clicking on the case, it opens a party selector and defaults to the first party added to the case. In some cases it could be the other party rather than the client. If someone clicks on the case, then clicks on the Select button it will tag the message in the Re: Client field to someone whose not even the client.
The suggestion is to change the order of the parties to the order on the party tab rather than the order the parties were added to the case.