In https://github.com/apache/kafka/pull/290, we added the logic to cache all brokers (id and ip) that the client has ever seen. If we can't find an available broker from the current Metadata, we will pick a broker that we have ever seen (in NetworkClient.leastLoadedNode()).
One potential problem this logic can introduce is the following. Suppose that we have a broker with id 1 in a Kafka cluster. A producer client remembers this broker in nodesEverSeen. At some point, we bring down this broker and use the host in a different Kafka cluster. Then, the producer client uses this broker from nodesEverSeen to refresh metadata. It will find the metadata in a different Kafka cluster and start producing data there.