HN Bug? I randomly have no downvote buttons on comments

I just noticed that on a random story in the comments I had some comments that I could up-/downvote (like I am supposed to) but many I could only upvote. Screenshot: http://i.imgur.com/O51Z0.png (from somewhere on http://news.ycombinator.com/item?id=3743693 )

I know there is some "you cannot downvote direct replies to your own comments" but never noticed anything like this. Is it a bug or some undocumented confusing feature?