The problem they had in the link provided by Dennis is that they performed the tasks in an incorrect order. What they did was power on the new primary first without connecting it to the network, then configured failover on the primary. At this point the primary thinks it is the only ASA that is alive in the pair (and so does the standby.) when the primary was connected to the network the standby sees that the primary has been active and assumes the standby role.
This is what should have been done.
1. install the ASA in the rack
2. connect all network cables
3. power on the ASA
4. configure failover on the primary ASA
5. issue no shut on the failover interface
Now when the primary comes online, it checks to see if there is already and active ASA, it sees that the secondary is the active ASA and then the primary assumes the standby role. It is important to remember that the ASA that becomes the active is not always the ASA configured as primary. Lets say you configure two ASAs for active/standby in a staging area, power them down, install them at the site and then power them on at the same time. Now, the first ASA to come online will become the active. So lets say that the secondary boots faster than the primary, then the secondary will become the active ASA. So always power on the ASA that is to become the active first, and then the secondary.
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