The ORA-609 error is thrown when a client connection of any kind failed to complete or aborted the connection
process before the server process was completely spawned.
Beginning with 10gR2, a default value for inbound connect timeout has been set at 60 seconds.
This is also triggered, when a DB session is killed/aborted manually from the OS prompt.
Increase the values for INBOUND_CONNECT_TIMEOUT at both listener and server side sqlnet.ora file as a preventive measure.
If the problem is due to connection timeouts,an increase in the following parameters should eliminate or reduce the occurrence of the ORA-609s.