WARNING: inbound connection timed out (ORA-3136)
                                                 Last update (2012-06-19 10:15:03)
                                                                                                                   Date added (2012-06-19 10:00:28)

Summary
In the alert log of an Oracle10g database i get errors like:

WARNING: inbound connection timed out (ORA-3136)
Tue Jun 19 07:56:46 2012
WARNING: inbound connection timed out (ORA-3136)
Tue Jun 19 08:02:14 2012
WARNING: inbound connection timed out (ORA-3136)
Tue Jun 19 08:03:51 2012
WARNING: inbound connection timed out (ORA-3136)

The most common root cause of "WARNING: inbound connection timed out (ORA-3136)" is
ORA-01017 invalid username /password

Some users from their client (sqlplus, toad, etc) are trying to connect to the database but after wrong username/password, they leave the client application open.

After x seconds (set at parameter INBOUND_CONNECT_TIMEOUT at listener.ora or
SQLNET.INBOUND_CONNECT_TIMEOUT at sqlnet.ora) the message:
WARNING: inbound connection timed out (ORA-3136) appears in the alert.log

Workaround: Set at sqlnet.ora SQLNET.INBOUND_CONNECT_TIMEOUT=0
This means it will wait indefinite to alert the warning.