We ran into the following sequence of events:
- master startup failed after only ROOT had been assigned (for another reason)
- restarted the master without restarting other servers. Since there was at least one region assigned, it went through the failover code path
- master scanned META and inserted every region into /hbase/unassigned in ZK.
- then, it called "listChildren" on the /hbase/unassigned znode, and crashed with "Packet len6080218 is out of range!" since the IPC response was larger than the default maximum.