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

ORACLE ERROR CODE ORA-00018
ERROR
maximum number of sessions exceeded
CAUSE
All session state objects are in use
ACTION
Increase the value of the SESSIONS initialization parameter

ORACLE ERROR CODE ORA-00017
ERROR
session requested to set trace event
CAUSE
The current session was requested to set a trace event by another session
ACTION
This is used internally; no action is required