What does `max.poll.interval.ms` actually protect against, and what happens if a consumer exceeds it?

It's the maximum time allowed between successive calls to `poll()` — if your processing logic between polls takes longer than this (e.g. a slow downstream call), Kafka assumes the consumer instance is stuck or dead and triggers a rebalance, reassigning its partitions to other consumers, which can cause the 'stuck' consumer to have its work duplicated once it finally does respond.