The standby unit might crash upon the bootup if the invalid information is passed from the active unit in stateful update.
Perform these steps as a workaround:
The problem has been resolved in PIX version 6.3(4) and 6.3(3.137).
For more information, refer to Cisco bug ID CSCee18849.
For more information on configuring failover in the PIX firewall, refer Using PIX Firewall Failover.
For more information on configuring failover in Cisco Security Appliance, refer to the Configuring Failover section of CSA command line configuration.