Oracle Errors Interview Question:

ORA-12509: TNS:listener failed to redirect client to service handler?

Submitted by: Administrator

Cause: The dispatcher terminated unexpectedly

Action: Attempt to connect again and if the same error
occurs, contact the DBA to check the state of the
dispatchers for this SID. If the problem persists, turn on
tracing in the listener to determine the TNS error caused by
the redirect.
Submitted by: Administrator