ORA-01012

This was tested on Oracle 9. If you are in the middle of a shutdown immediate in one session (in red): 

SQL> shutdown immediate

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> 

... and you connect as SYS in another session (in blue), the session starts without an error message but, if you run any SQL, you get an ORA-01012: 

ANDREW > sqlplus '/ as sysdba'

 

SQL*Plus: Release 9.2.0.5.0 - Production on Wed Jun 6 14:35:47 2012

 

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

 

Connected.

SQL> select sysdate from dual;

select sysdate from dual

*

ERROR at line 1:

ORA-01012: not logged on

 

SQL>