If you’re using AGs, don’t apply these patches:
- SQL 2012 SP2 CU3
- SQL 2012 SP2 CU4
- SQL 2014 CU5
until you read this Microsoft post about a breaking bug.
Your AG may stop synchronizing due to blocking between user queries and a system session. The fix is to disable automatic failover, restart the primary, and enable automatic failover again.