Trivial Description
If KafkaConsumer.position is called with a partition that the consumer isn't assigned, it throws an IllegalArgumentException. All other APIs throw an IllegalStateException when the consumer tries to act on a partition that is not assigned to the consumer.
Looking at the implementation, if it weren't for subscription test and IllegalArgumentException thrown at the beginning of KafkaConsumer.position, the very next line would throw an IllegalStateException anyway.