Recently I saw some clusters that are not patched and or where not aware that there were hotfixes.
So here is a list of required and optional Windows 2008R2 SP1 hotfixes.
This list is found on several pages on the web and could be handy.
Windows & Hyper-V : Required Hotfixes
Validate SCSI Device Vital Product Data (VPD) test fails after you install Windows Server 2008 R2 SP1
http://support.microsoft.com/kb/2531907 (required for 3+ node Hyper-V clusters)
The network connection of a running Hyper-V virtual machine may be lost under heavy outgoing network traffic on a computer that is running Windows Server 2008 R2 SP1
The Cluster service stops unexpectedly on a Windows Server 2008 R2 failover cluster node when you perform multiple backup operations in parallel on a cluster shared volume
MPIO failover fails on a computer that is running Windows Server 2008 R2
The MPIO driver fails over all paths incorrectly when a transient single failure occurs in Windows Server 2008 or in Windows Server 2008 R2
Performance decreases in Windows Server 2008 R2 when the Hyper-V role is installed on a computer that uses Intel Westmere or Sandy Bridge processors
Stop error 0x0000007a occurs on a virtual machine that is running on a Windows Server 2008 R2-based failover cluster with a cluster shared volume, and the state of the CSV is switched to redirected access.
An update is available for Hyper-V Best Practices Analyzer for Windows Server 2008 R2
“0x0000009E” Stop error when you add an extra storage disk to a failover cluster in Windows Server 2008 R2
A virtual machine online backup fails in Windows Server 2008 R2 when the SAN policy is set to Offline All
Cluster node cannot rejoin the cluster after the node is restarted or removed from the cluster in Windows Server 2008 R2
Cluster service stops when an error occurs in the registry replication process of a failover cluster in Windows Server 2008 R2 or in Windows Server 2008
0×20001 Stop error when you start a Linux VM in Windows Server 2008 R2 SP1
A heap memory leak occurs when an application or service queries the MSCluster_Resource WMI class in Windows Server 2008 R2
Cluster service initiates a failover after a delay of about 80 seconds when you shutdown the active node in Windows Server 2008 R2
New registration entries are added to the Persistent Reservation table when the physical disk resource that is associated with the CSV is taken offline on a Windows Server 2008 R2-based Failover Cluster
A transient communication failure causes a Windows Server 2008 R2 failover cluster to stop working
Cluster service leaks memory when the service handles state change notifications in Windows Server 2008 R2 or Windows Server 2008
Hyper-V Export function consumes all available memory in Windows Server 2008 or in Windows Server 2008 R2
Microcode update for Intel processors in Windows 7 or in Windows Server 2008 R2
Corrupted VSS snapshot