I have seen today this warning into a listener log;
WARNING: Subscription for node down event still pending
These messages are related to the Oracle TNS Listener’s default subscription to the Oracle Notification Service (ONS). In a non-RAC environment it is recommended to disable this subscription.
You can disable this feature by set the nex parameter into your listener.ora file.
For more information, you can refer to MOS, note 372959.1
HTH – Antonio NAVARRO