I updated my production SQL Server 2012 cluster from SP1 to SP3 CU1 last night and had to spend an hour trying to figure out why the SQL Server Engine service wouldn't start for one of my two instances. Weirdly the other instance worked fine after the upgrade. The error logs were no help at all. It was only by the magic of the gods that I happened upon a registry entry that still had the old patch level in it.HKEY_LOCAL_MACHINE\SOFTW... SQL Server\<instance>\Clu... ......