Solution:
as @robsteady mentioned here, there is a +/- button that only appears on hovering over the hidden replies, so it's still possible to show the voting icons and make actions.
---------- original post and earlier edits -----------------
If accidentally down-voted a reply (not a top post), it will be hidden, so unable to see the content, nor change vote.
What if there is a bot down-voting every reply in first time?
//Edit:
An example: in this post @liondani wrote that author perhaps made a fake account , then it got down-voted by the author. Fortunately someone else up-voted that again (which I think is not done on steemit.com), so it re-appeared.
//Edit2:
For those who wants to up-vote a hidden comment, a work-around right now, is go to the author's Posts page, it's still possible to up-vote there.
//Edit3:
And worse, other replies to the hidden reply get hidden as well.