I did SQL 2005 SP4 patch for 4 SQL instances on 3 nodes cluster this morning. The patch on one SQL instance was failed due to permission issue .
I login with my adm account which is in local admin group. We have done on test and DR box before and no problem at all. I had to use cluster admin tool to start the sql instance. Actually it has been patched to SP4 after I checked. Then I did failover the
instance to another node. The server authentication was changed to windows mode only, all SQL logins failed to access databases. So I had to change the SQL to mix authentication mode and restart the instance.