What Happens If You Use a Wrong Connect Identifier? - Oracle DBA FAQ - Introduction to Command-Line SQL*Plus Client Tool

Of course, you will get an error, if you use a wrong connect identifier. Here is an example of how SQL*Plus react to a wrong connect identifier:

SQL> CONNECT fyi/retneciyf@WRONG;
ERROR:
ORA-12154: TNS:could not resolve the connect identifier
   specified

Warning: You are no longer connected to ORACLE.

What you need to do in this case:

• Check the CONNECT command to make sure that the connect identifier is entered correctly.
• Check the tnsnames.ora file to make sure that the connect identifier is defined correctly.
• Check the tnsnames.ora file to make sure that there is no multiple definitions of the same connect identifier.
• Check your files system to see if you have multiple copies of tnsnames.ora in different Oracle home directories, because you installed multiple versions of Oracle. If you do have multiple copies, make sure your SQL*Plus session is picking up the correct copy of tnsnames.ora.