Oracle: How to cancel a pending 'shutdown immediate'

[ATC]

Idle clients are preventing 'shutdown immediate' from actually shutting doen the instance. Issuing another command like 'shutdown abort' fails with a string of error messages:
ORA-24324: service handle not initialized
ORA-24323: value not allowed
ORA-01089: immediate shutdown in progress - no operations are permitted

[ATC]

Although you cannot cancel the shutdown per se, you can restart the instance by connecting as sysdba and issuing the following command:
'startup force'

From the shutdown pending state, this actually causes the instance to 'shutdown abort' and then start up, minimizing downtime.