Recently went to apply Service Pack 2 to a SQL Server 2012 active/passive cluster, and it failed with the error message, Cluster Group cannot be determined for the Instance named Removed This indicates there is a problem with the product registry.
So after scratching my head for two hours, and now outside my maintenance window, I read the message literally and found the ClusterName in the registry did not match what was running in ClusterManager.
Why this happened is because this cluster was built with a "temporary" name and then the virtual name swapped with another cluster when we upgraded from sql 2008 to 2012. Now why when we moved the virtual name it didn't update the registry I don't know.
The fix was to manually update the registry (I love doing that!), re-run service pack 2 and all was fine.