Which scenario will cause automatic high availability (HA) synchronization to fail?

 According to the Citrix documentation, automatic HA synchronization will fail if the primary and secondary Citrix ADCs have different build versions. The build versions must be the same for the configuration to be synchronized. A manual synchronization, a configuration change to the primary Citrix ADC, or a force failover will not affect the automatic HA synchronization1

1: Citrix Virtual Apps and Desktops 7 Deployment and Administration - High Availability