When a stanza is sent from one occupant to all other occupants in a MUC room, the 'from' address of the stanza that is sent to all occupants should be a JID of which the bare JID equivalent matches the JID of the room. This, however, does not seem to always be the case.
A possible symptom of this problem is messages arriving at occupants, being displayed outside of the context of a chat room.
This issue has been observed only recently, after clustering was enabled and used - and even then, it's hard to reproduce.
Recently, a check was added that would log an exception when the 'from' address does not match the expected pattern (this change prevents the symptom described above, as it drops the message). This caused this stack trace to be logged: