The client can only find out if the server has disconnected when it tries to
read or send something. If the server gets disconnected before
send_client_reply_packet(), the client will try sending authentication
information but it will fail. But, if the client is fast enough to send
autentication information before disconnecting, it will notice that when
reading the ok packet. So the client can fail on read or on write.
It is unpredictable because, the process are unsynchronized and this
could happen in any order.