It is a bit of splitting hairs in some sense, but the key concept here is just because the message was delivered to the Kafka client successfully, does not mean

It is a bit of splitting hairs in some sense, but the key concept here is just because the message was delivered to the Kafka client successfully, does not mean it was processed by the application. You will have to explicitly ack if you want that guarantee. For a concrete example, lets say all you do with a message is write it to a database. As soon as that message is in your client handler callback, that message is ack'ed. But you probably only want that ack to happen after a successful insert into the DB. The most likely scenario here to cause unprocessed messages is that the DB is down for whatever reason (maybe a network link is down, or k8s or even a firewall config now prevents you from accessing), and at some point during this your client goes down, maybe by an eng attempting a restart to see if the problem goes away.