There are several posters on Less Wrong whom I

• think are unusually smart, and would probably test within the top 2% in the US on standardized tests, and
• think are usually wrong when they post or comment here.

So I think they are exceptionally smart people whose judgement is consistently worse than if they flipped a coin.

How probable is this?

Some theories:

• This is a statistical anomaly.  With enough posters, some will by chance always disagree with me.  To test this hypothesis, I can write down my list of smart people who are usually wrong, then make a new list based on the next six months of LessWrong, and see how well the lists agree.  I have already done this.  They agree above chance.
• I remember times when people disagree with me better than times when they agree with me.  To test this, I should make a list of smart people, and count the number of times I votes their comments up and down.  (It would be really nice if the website could report this to me.)
• This is a bias learned from an initial statistical anomaly.  The first time I made my list, I became prejudiced against everything those people wrote.  This could be tested using the anti-kibitzer.
• I have poor judgement on who is smart, and they are actually stupid people.  I'm not interested in testing this hypothesis.
• What I am actually detecting is smart people who have strong opinions on, and are likely to comment on, areas where I am either wrong, or have a minority opinon.
• These people comment only on difficult, controversial issues which are selected as issues where people perform worse than random.
• Many of these comments are in response to comments or posts I made, which I made only because I thought they were interesting because I already disagreed with smart people about the answers.
• Intelligence does not correlate highly with judgement.  Opinions are primarily formed to be compatible with pre-existing opinions, and therefore historical accident is more significant than intelligence in forming opinons.  The distribution of historical accidents is probably such that some number of smart people will have opinions based on an entire worldview that is largely wrong.