The script used to be

shutdown abort

it has been replaced by

#shutdown abort
shutdown immediate

Let’s try !

SQL> #shutdown abort
ORACLE instance shut down.
SQL> shutdown immediate
ORA-01012: not logged on

sqlplus just silently ignored the # symbol and executed the first statement.

Thanks to Maxim comment, here is a new case to explain the sql prefix #

SQL> sho sqlpre
sqlprefix "#" (hex 23)
SQL> select
  2  #prompt hello world
hello world
  2  * from dual;

D
-
X

While within (or outside of) an sqlplus block, you can tell sqlplus to immediately run a sqlplus statement

The correct syntaxes to put comment are documented in Placing Comments in Scripts

SQL> remark shutdown abort
SQL> rem shutdown abort
SQL> -- shu abort
SQL> /* shutdown abort */