ORACLE ERROR CODE ORA-00028
ERROR
your session has been killed
CAUSE
A privileged user has killed your session and you are no longer logged on to the database
ACTION
Login again if you wish to continue working

ORACLE ERROR CODE ORA-00027
ERROR
cannot kill current session
CAUSE
Attempted to use ALTER SYSTEM KILL SESSION to kill the current session
ACTION
None

ORACLE ERROR CODE ORA-00026
ERROR
missing or invalid session ID
CAUSE
Missing or invalid session ID string for ALTER SYSTEM KILL SESSION
ACTION
Retry with a valid session ID

ORACLE ERROR CODE ORA-00025
ERROR
failed to allocate string
CAUSE
Out of memory
ACTION
Restart with larger sga heap

ORACLE ERROR CODE ORA-00024
ERROR
logins from more than one process not allowed in single-process mode
CAUSE
Trying to login more than once from different processes for ORACLE started in single-process mode
ACTION
Logoff from the other process

ORACLE ERROR CODE ORA-00023
ERROR
session references process private memory; cannot detach session
CAUSE
An attempt was made to detach the current session when it contains references to process private memory
ACTION
A session may contain references to process memory (PGA) if it has an open network connection, a very large context area, or operating system privileges. To allow the detach, it may be necessary to close the session’s database links and/or cursors. Detaching a session with operating system privileges is always disallowed

ORACLE ERROR CODE ORA-00022
ERROR
invalid session ID; access denied
CAUSE
Either the session specified does not exist or the caller does not have the privilege to access it
ACTION
Specify a valid session ID that you have privilege to access, that is either you own it or you have the CHANGE_USER privilege

ORACLE ERROR CODE ORA-00021
ERROR
session attached to some other process; cannot switch session
CAUSE
The user session is currently used by others
ACTION
Do not switch to a session attached to some other process

ORACLE ERROR CODE ORA-00020
ERROR
maximum number of processes (string) exceeded
CAUSE
All process state objects are in use
ACTION
Increase the value of the PROCESSES initialization parameter

ORACLE ERROR CODE ORA-00019
ERROR
maximum number of session licenses exceeded
CAUSE
All licenses are in use
ACTION
Increase the value of the LICENSE MAX SESSIONS initialization parameter