When the ConnectionAcceptOrDeny message from the server is dropped on the way to the client it's possible for the server to end up deadlocked waiting for the client to continue the joining process. See the annotated image of the packet capture I've attached.
The server starts sending SynchronizerActions to the client immediately after it has sent the ConnectionAcceptOrDeny message, but if the client doesn't receive the ConnectionAcceptOrDeny message it waits half a second and then resends the ConnectioRequestReplyConfirm message it sent to the server.
In response to the second ConnectionRequestReplyConfirm message the server sends a modified ConnectionAcceptOrDeny message back with a higher first sequence number, if the client continues from this sequence then it will miss all the synchronizer actions sent by the server between the first and second ConnectionAcceptOrDeny message.
If the client is waiting for one of those synchronizer actions then the client and server deadlocks, the client is waiting for the server to send a message it has already sent, and the server is waiting for the client to signal it's finished downloading the auxiliary data. For players connected to the server this shows up as a progress bar at 100% saying "Server (<server>) is saving the map for player.", for the player connecting it's a progress bar at 100% saying "Waiting for the server to save the map.". The server remains frozen until the player connecting aborts the connection.
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