Today I was researching something on My Oracle Support, and I ran across document ID 1405209.1 detailing a scenario where Exadata ZFS appliances won't be able to rejoin the cluster due to a software update if you have certain network cards installed. So what you ask? Well, other than the obvious research you want to do if you are on Exadata, if you don't get to play with a lot of commands for Exadata this has several that might be useful if you encounter this message:
This appliance is part of an appliance cluster.
Please wait while cluster synchronization takes place.
Joining cluster ... /