ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

If you get errors similar to this:

SQL> conn / as sysdba
ERROR:
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

The cause of this issue is the fact that the environment varialble TWO_TASK is visible. You need to unset TWO_TASK and try sqlplus again.