If the session is not making a SQL call to Oracle (is INACTIVE) when it is
terminated, the ORA-28 message is not returned immediately. The message is not
returned until the user subsequently attempts to use the terminated session.
When an inactive session has been terminated, STATUS in the view V$SESSION is
“KILLED.” The row for the terminated session is removed from V$SESSION after
the user attempts to use the session again and receives the ORA-28 message.
If an active session cannot be interrupted (for example, it is performing
network I/O or rolling back a transaction), the session cannot be terminated
until the operation completes. In this case, the session holds all resources
until it is terminated. Additionally, the session that issues the ALTER SYSTEM
statement to terminate a session waits up to 60 seconds for the session to
be terminated;