FIX: Event ID: 21421 occurs when you use the Microsoft Operations Manager 2005 agent to monitor a clustered instance of SQL Server 2005 in a clustered environment
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Source: Knowledge Base
Product: Microsoft SQL Server 2005 Standard Edition, SQL Server 2005 Enterprise X64 Edition & SQL Server 2005 Enterprise Edition
Notification Contents: New and All Modifications
FIX: Event ID: 21421 occurs when you use the Microsoft Operations Manager 2005 agent to monitor a clustered instance of SQL Server 2005 in a clustered environment
http://support.microsoft.com/kb/938911/en-US