Kafka partition lag grows unbounded

A Kafka consumer group (Java) processes an events topic with 12 partitions, in-order per partition. At 13:00 consumer lag on exactly one partition starts growing unboundedly while the other 11 partitions stay at near-zero lag. Throughput on that one partition collapsed. Logs from the consumer assigned to that partition show it repeatedly trying to process the same message offset, throwing, and retrying, it never commits past that offset. A producer started emitting a new event type at 12:55. Triage and mitigate.