Fatal_parse_failure happens when we receive data that does not conform to the Msgpack specification. Parsing stops and if close_reader_and_writer_on_disconnect was true when connect was called, reader and writer will be closed.

Invalid_rpc_message happens when we receive valid Msgpack data that does not conform to the Msgpack RPC protocol.

Response_for_unknown_request happens when we receive a message that is meant to be a response to a synchronous request we sent but we don't have a record of that request (i.e., the response's message ID is unknown).

Unknown_method_called happens when we receive a synchronous request for a method that has not yet been registered.