Keith Parsons at "The Secular Outpost" has a list (ht) of twenty "stupidest things philosophers have said over the millennia". With a few exceptions, it reads more like a list of twenty common myths about what philosophers believe; and, except for #19, which really was stupid, most of them were not stupid things to say at all, even if wrong. (I'm also a little puzzled that stupid claims about women are included, but no stupid claims about blacks, e.g., Hume's "There never was a civilized nation of any other complexion than white, nor even any individual eminent either in action or speculation," which was an extraordinarily stupid thing to say, as Beattie pointed out at the time.) Some notable misunderstandings that are just obvious from first glance:
(1) Berkeley does say that matter doesn't exist; but by 'matter' he is quite clear that he means something very specific, namely, the "something, I know not what" of Locke and others. Matter in the sense studied by physics is something different; and Berkeley recognizes that there are other senses in which you could take the term.
(2) Locke doesn't say anything that commits him to the claim that atheists are untrustworthy; he is committed to the related, but still somewhat different, claim that they have no consistent reason to be where promises, contracts, and oaths are concerned -- not that they don't keep their promises, which would depend entirely on the individual atheist, but that 'promises have no hold on them'. Locke does in fact think you can have a society of atheists; it is implausible to suggest that he thinks you can have a society of people who are in fact always untrustworthy. He would just be committed, at least in the terms he uses in A Letter Concerning Toleration to saying that such a society had no rationally consistent ground for promising.
(3) Zeno, contrary to popular belief, does not argue that change is impossible. He argues that either change is impossible or it is not really possible for physical things to be infinitely divisible -- that is, he argues that if physical space is infinitely divisible, change is impossible. There is some (although perhaps not completely conclusive) reason to think that the infinite divisibility claim is the conclusion he actually wanted people to reject.
(4) Leibniz says that this world is the best of all possible worlds, but not in the sense Voltaire attributed to him, since he meant something very precise, namely, that it is impossible for God to create a world that involves both simpler means and richer diversity of effects than this one.
(5) Spinoza holds that things could not be otherwise than they are for a reason, namely, that he argues that the claim that there are contingent things (as opposed to the claim that everything logically implies everything else) requires affirming a doctrine of final causes, since efficient causes are purely necessary and only final causality is slippery enough to leave possibilities not ruled out. I think he's wrong (about the nonexistence of final causes) but it's not a stupid argument.
(6) Wittgenstein does not say that we can't know pains; he says that we can't know pains if we have to think of them in terms of a "model of 'object and designation'" or, to put it in other terms, if they are purely private objects designated by language. Obviously the conclusion is that we should not think of them in that way.
(7) The Churchlands don't say that there are no pains; eliminative materialism is not the claim that we aren't referring to anything when we talk about pains (or mental phenomena). If you're an eliminative materialist about pain, your view is that there is no particular phenomenon referred to by the term 'pain'; this doesn't mean that we never refer to anything using the term, only that 'pain' is not a class in a completely natural classification of things in the world. If you are an eliminative materialist about belief, you think that 'belief' is just a crude and approximate way of talking about the brain, good enough for practical purposes now, but not something you will find in a completely adequate scientific account of the things we're referring to when talking about belief. And so forth.
(8) Quine does not say that "When someone speaks in a native tongue that is historically unrelated to yours, you can never know what he or she means." Again, a number of conditions and qualifications have been dropped. He claims that linguistic behavior underdetermines meaning. This is a very different claim; he holds that you can know what other people mean, but it's not by picking out a unique and uniquely determinable meaning that the words must be conveying.
(9) It is absolutely absurd, absolutely absurd, to attribute the claim "You cannot learn from experience" to Hume. Hume is an empiricist! He thinks that everything we learn is learned from experience (and yes, he does think we learn things)! If someone is going to say something like this, one is tempted to start a list of stupid things they have said, with this at the top of the list.
But lists like these are useful in any case; they help us historians of philosophy to know where we have to do a better job communicating.
[ADDED LATER: I should say as well that not only is #20 not obviously false, it is arguably the most reasonable position on the subject; the only other coherent account of evil is one in which good is a privation of evil, which is problematic for all sorts of reasons. No other account that has been proposed has ever withstood as much serious scrutiny. Whether you are atheist or theist, Aristotelian or not, a broadly Augustinian account appears to be the most reasonable account of evil. (I say 'broadly Augustinian' because one can argue -- it is a question that gets into fields in which I am not competent to say whether the arguments are successful -- that Mencius also takes evil to be a privation, and if so, he obviously does so independently.)]