It appears that a recent update has introduced a design flaw: on posts that have had comments that have all been deleted, a gray horizontal line appears above the "add comment" button a few seconds after it loads. Here's an example post:
But on this post, which (to my knowledge) has never had any comments posted on it before, not even deleted ones, doesn't show such a horizontal line.
While it probably is a good idea to indicate publicly that comments have been posted on the post, but have all been deleted, I think this line is probably an unintentional design bug. Also, in my opinion, any such intentional indicator should be much more conspicuous.
Reproduced under both Internet Explorer 11 and Chrome on Windows 8.1.
Update: This appears to happen only when following a link to a deleted comment, not when following a link to the post itself. When following a link to a deleted comment on a post that has had all its comments removed, the line is visible; otherwise, when following a link to the post itself, it's not. The line appears a few seconds after the post has loaded.