Comments in SQL*Plus

This Warning from Eddie Awad remind me of problem with comments I've had some time ago. I've prepared an script that was intended to run in sqlplus. Then I've extensively tested it, and as a last step, I've commented it. And ... did not test again. When this script was run on production caused errors. The problem was in the way that sqlplus interpret comments.
Example:

SQL> select * from dual;

D
-
X

runs fine, while this:

SQL> select * from dual; -- some comments

stops at second line waiting for input:

SQL> select * from dual; -- some comment
  2 

It was really unpleasant experience.

Cheers, Paweł