If you have a query in the SQL Editor window that you have run, like:
Select * from Dual;
Then you lose your network connection (unplug your network cable for 30 seconds).
Then plug it back in. Then run the query (F9).
You will get a pop-up window ‘Not Connected. Reconnect?”. Press Yes.
After it reconnects, you get an error - ORA-12571: TNS:packet writer failure.
Then if you press F9 again, it works fine. It just seems to miss something when it reconnects before it runs the query.