If a SUBSCRIBE, SUBACK, UNSUBSCRIBE or UNSUBACK message is lost during transfer, the packet identifier is leaked. The leak remains for the duration of the session, which in the case of CleanSession=0 could be indefinite, during which time additional loss of packet ids could occur.

This cannot be fixed using the existing mechanisms, and is recorded here for future consideration (not MQTT 3.1.1. or MQTT 5.0).