What's a common misconfiguration when running a packaged Kafka consumer app standalone (outside Docker Compose) for the first time?

Pointing `bootstrap.servers` at `localhost:9092` when the app is actually running in an environment where Kafka is reachable at a different hostname/port (e.g. a Docker service name or a remote broker) — this is one of the most common 'works on my machine but not elsewhere' Kafka connectivity bugs, since the client fails silently-ish with connection retries rather than a clear error.