I believe I have two or three "upvotes" in my voting history where I meant to click parent. I'm pretty sure I was not authenticated then, so maybe those upvotes were discarded, or maybe not...
It's perhaps also worth noting that the "bogus upvote" problem is in some way related to some other social problems which we struggle with. Spam, and the wrong person using a password, both seem to me to be a reflection of social problems reflected by system design.
For spam, there's no way to ask the supposed origin mail server "is this from:, to:, message-id: combo a plausibly legit in-flight message?" Because some systems cannot handle that functionality efficiently, mail standards people have rejected it (and similar approaches, perhaps identifying a sequence of mail servers, perhaps involving multiple queries when mailing lists are involved) for the multitude of cases where it makes sense. But note that you do not need perfection here, to massively reduce spam volume. Note also that there are efficiency arguments, here, but the original protocol worked fine on machines with <64k memory and ~1MHz clocks.
Passwords and authentication protocols and buffer overflows are a more complex issue, and I'm drifting dangerously off subject already. But that's another issue, I think, where "design efficiencies" and "overly focussed thinking" sometimes bite us. (And perhaps sometimes benefit us, but how can we even measure this?)
Anyways, trying to get back on topic, personally, I have little faith in upvotes here, not even my own, so mostly I rely on friends to point out if there's anything worthwhile here, for me to read.
It's a triangle, someone clicked on it. Counters are a good first pass, for filtering bulk raw information and indicating things to focus on, but they are not the end-all and be-all for good ui design. Borrowing wholesale from stack overflow might be good, or maybe some variant would also be plausible (like, maybe: when replying, let people with parent posts weight child posts under theirs that they found helpful, giving some fraction of the total weight of upvotes in any child posts of their parent post to posts they want to highlight. This should be adjustable, in some simple way, or we live with accidents. There's also an issue when one person has multiple parents but maybe it's best to default to the top level eligible parent.)
My above parenthetical suggestion is not perfect, but it might help for distinguishing between some sincere and insincere replies. Another approach might be to give the user some "downvote" ability - abiding by the original site design, these need not be preserved for others (it's a personal decision, after all).
Ultimately, to really get this right, I would want to be able to segregate other voters into "agree" / "disagree" / "irrelevant" buckets based on a label which reflected my current point of view, and have the content displayed based on that categorization. But that's a much harder problem.