This is not the biggest flaw! It was that messages that are in flight (sent, but not yet delivered) still get delivered if the key changes. Only after delivery does the sender get notified of the key change. The last time, I gave example where a protest organizer (Bob) is detained by the (secret) police in the period leading up to a protest, and denied access to their password-protected phone that's switched off. All the police has to do is wait for a while, then pop the SIM into a new phone and presto, they get all the incriminating messages that were sent by co-conspirator Alice while Bob was detained. Only after delivery onto the police phone does Alice get notified that Bob's key has changed.