I've only seen it happen occasionally, but when it does happen, it's consistent for the occurrence, well, at least for a little while. It does not last for very

I've only seen it happen occasionally, but when it does happen, it's consistent for the occurrence, well, at least for a little while. It does not last for very long, so there's a time element involved, similar to how you get a "Can't make that vote" error if you try to up-vote something that is too old. It might be a bug caused by how caching is handled, or it might be intentional, either way, I've never actually tested to see if the odd down-vote button on a parent post actually works.