The DocBlock comments on
comment_reply() document the return value thus:
* The rendered parent node or comment plus the new comment form.
That's not correct. What's actually returned is an array containing a renderable array for the node or parent comment and the comment form as an array. I propose the comment be changed to read something like this:
* @return array
* An associative array containing:
* - An array for rendering the node or parent comment.
* - comment_node: If the comment is a reply to the node.
* - comment_parent: If the comment is a reply to another comment.
* - comment_form: The comment form as a renderable array.
When there's consensus on the change I'll roll a patch.