When we stopped the oracle service, one of the clients got the error: "ORA-00028: your session has been killed" and then
instead of: "ORA-03114: not connected to ORACLE"
it got: "ORA-01012: not logged on"
That means that the connection stood alive.
Similar things happen when you execute "ALTER SYSTEM KILL SESSION" on an inactive connection
(killing connections that are, for example, performing SELECTs at the moment, disconnects them)
The fact that a connection may stay alive after the error "your session has been killed" means that clients that get this error should close the connection themselves.