In Classic 1.5 theme, comments.php has a bug.
When there is no comments, a message is shown 'No comments yet.'
That's OK for most cases, but,
If a post does not accept comments, it is also shown 'No comments yet.'
This occurs because the number of comments is checked, as shown below.
Line 17: <?php if ( $comments ) : ?>
...
Line 31: <?php else : // If there are no comments yet ?>
Line 32: <?php _e('No comments yet.'); ?>
Line 33: <?php endif; ?>
I think 'Sorry, the comment form is closed at this time.' is a better message, if comments are NOT accepted.