Oracle Errors Interview Question:

ORA-12521: TNS:listener does not currently know of instance requested in connect descriptor?

Submitted by: Administrator

Cause: The listener received a request to establish a
connection to a database or other service. The connect
descriptor received by the listener specified in addition to
the service name an instance name for an instance (usually a
database instance) that either has not yet dynamically
registered with the listener or has not been statically
configured for the listener. This may be a temporary
condition such as after the listener has started, but before
the database instance has registered with the listener.
Submitted by: Administrator