Conspiracy theories are very enticing to the human mind, so they often get too many upvotes here on HN. Although I did downvote pg's first comment in the thread mentioned here, I was outnumbered by many more people who upvoted it, mistakenly.
Top comments in a thread frame discussion - if the top comment in a thread is a conspiracy theory, then it's likely that the entire discussion will be derailed. For that reason, I think it's valid to downvote such comments, to prevent them from derailing the discussion.
I downvoted a recent comment by pg that for quite a while was (and STILL is) the top comment in its thread
http://news.ycombinator.com/item?id=4253156
but that pg later acknowledged didn't have a sufficient factual basis.
http://news.ycombinator.com/item?id=4254592
Conspiracy theories are very enticing to the human mind, so they often get too many upvotes here on HN. Although I did downvote pg's first comment in the thread mentioned here, I was outnumbered by many more people who upvoted it, mistakenly.