So, the reason we wouldn't fall for that one is that the therapy wouldn't pass the safety tests required by first-world governments. We have safety tests for all sorts of new technologies, with the stringency of the tests depending on the kind of technology — some testing for children's toys, more testing for drugs, hopefully more testing for permanent cognitive enhancement. It seems like these tests should protect us from a Question-Answerer as much as from human mistakes.

Actual unfriendly AI seems scarier because it could try to pass our safety tests, in addition to accomplishing its terminal goals. But a Question-Answerer designing something that passes all the tests and nevertheless causes disaster seems about as likely as a well-intentioned but not completely competent human doing the same.

I guess I should have asked for a disaster involving a Question-Answerer which is more plausible than the same scenario with the AI replaced by a human.