Sam Harris is here offering a substantial amount of money to anyone who can show a flaw in the philosophy of 'The Moral Landscape' in 1000 word or less, or at least the best attempt.
http://www.samharris.org/blog/item/the-moral-landscape-challenge1
Up to $20,000 is on offer, although that's only if you change his mind. Whilst we know that this is very difficult, note how few people offer large sums of money for the privelage of being disproven.
In case anyone does win, I will remind you that this site is created and maintained by people who work at MIRI and CFAR, which rely on outside donations, and with whom I am not affiliated.
Note: Is this misplaced in Discussion? I imagine that it could be easily overlooked in an open thread by the sorts of people who would be able to use this information well?
The usual reason for doing so is signalling: look how sure I am of my ideas, I am willing to put my money on the line. Most people who see this offer (aptly called a "challenge") won't hear "he would be happy to be disproven, what a rational fellow"; they will hear "he is sure he can't be disproven, what a confident fellow".
I haven't read Harris's book and don't know anything about it. However, I do feel that a genuine "challenge" should have a formal verification procedure for proposed answers, or at least a third party to judge them. Judging answers by whether they convince Harris himself requires extremely high confidence in his skills as a rationalist, even apart from his incentives.
On the other hand, what purpose is served by publishing the best answer even if it fails to convince him? He may end up publishing an answer that he thinks is completely wrong (and necessarily saying so), and maybe most other people will think it's wrong too (but that some other answer is right). The submitter will be rewarded with 1000$ although he hadn't convinced anyone, and nobody will change their opinions.