CU10 is a must for all instances where mirroring is being used

If you happen to use Database Mirroring on SQL Server 2005, then Cumulative Update 10 for SQL Server 2005 Service Pack 3 is a must for your instances. There are 2 database mirroring related fixes in this cumulative package. I would suggest that all SQL Server 2005 instances that have database mirroring enabled need to have this update installed ASAP.

982933 ( )
Error message when you shrink data files on principal in a Database Mirroring for two SQL Server 2005 servers

983480 ( )
FIX: Various issues occur when a database is involved in a database mirroring session and in a transactional replication if trace flag 1448 is enabled

983500 ( )
FIX: The role switch is delayed when a mirroring automatic failover occurs in SQL Server 2005