The chat FAQ says here that:
Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. A room is considered worth retaining if it has more than 15 messages by at least 2 users.
This could be understood in two ways.
- At least two users posted in the room. The number of all messages posted in the room is at least 15. (From my experience so far, it seems that this is how it works.)
- There are at least two users, such that each of them posted at least 15 messages.
I don't think that this could cause problems too often, but perhaps it might be better if the FAQ is unambiguous.