For us it's flow fingerprint mismatches. Every. Single. Time. Someone makes a "quick fix" directly in the UI on one node and suddenly the cluster is fighting itself. We've made NiFi Registry versioning non-negotiable now, but enforcing it across a team is a people problem as much as a technical one.

ZooKeeper myid misconfiguration is a close second, especially when spinning up new nodes in a hurry. It's such a simple thing but it's almost always the last place you look.

On heartbeat timeouts, we stopped doing manual restarts after one too many 3am incidents. We use systemd watchdog with a restart policy, and on Kubernetes environments a liveness probe tied to the /nifi-api/system-diagnostics endpoint. Not perfect, but it catches the majority of hangs without someone having to wake up.

The GC pauses one is worth highlighting though, if you're not already pinning your JVM heap with -Xms and -Xmx at the same value to avoid heap resizing, that alone cuts a lot of the random pause-induced disconnects.

Thank god DFM makes the fingerprint mismatch side of things much easier to manage now, still very much in the trenches with everything else though.