Stateful failover has not been configured correctly.
For stateful failover to work properly, the stateful failover PIX interfaces (Primary/Secondary) must use a dedicated Ethernet interface on the PIX.
This dedicated Ethernet interface cannot be the inside or outside, or any other already-used interface. The Ethernet segment should not contain any hosts on it. A crossover cable can be used; however, it is recommended that the stateful failover interfaces be plugged into a switch.
If you use a switch, make sure that the switch ports to which you connect the PIX failover interfaces have the following properties.