It's about a happiness study, but it may be one of the better examples:
The gist is that people remember the end of an uncomfortable experience more than the time in between and map that feeling of a "happy end" onto the whole experience in retrospect. In one study two groups of subjects got a colonoscopy. Both groups were in considerable discomfort for equal amounts of time, when in group 1 the colonoscopy was terminated, while for group 2 the instrument wasn't removed immediately and thus caused considerably less discomfort as during the treatment, but obviously still more discomfort than in the group where it was removed entirely. When both groups were asked to evaluate their experience afterwards, the group which suffered discomfort for longer (but where the discomfort was curtailed down somewhat at the end) didn't think the experience was as bad, as the control group which suffered less on all accounts in comparison.
Unfortunately it may be a somewhat known finding by now, considering that you can find it in a TED-talk: http://www.ted.com/talks/lang/eng/daniel_kahneman_the_riddle_of_experience_vs_memory.html
Another problem with taking the study as an example may be, that it may feel almost too much like a set trap. If we didn't know the results, upon hearing the study setup we would think: "Well duh, of course the patients with less overall discomfort will report that they had a better experience" but that seems to be such an obviously and intuitively trivial conclusion that I'd get suspicious immediately if I encountered that example.
EDIT: By the way, are you planning to include a rough outline of evolutionary theory or include evolution in some other fashion Eliezer?
I'm writing the section of the rationality book dealing with hindsight bias, and I'd like to write my own, less racially charged and less America-specific, version of the Hindsight Devalues Science example - in the original, facts like "Better educated soldiers suffered more adjustment problems than less educated soldiers. (Intellectuals were less prepared for battle stresses than street-smart people.)" which is actually an inverted version of the truth, that still sounds plausible enough that people will try to explain it even though it's wrong.
I'm looking for facts that are experimentally verified and invertible, i.e., I can give five examples that are the opposite of the usual results without people catching on.
Divia (today's writing assistant) has suggested facts about marriage and facts about happiness as possible sources of examples, but neither of us can think of a good set of facts offhand and Googling didn't help me much. Five related facts would be nice, but failing that I'll just take five facts. My own brain just seems to be very bad at answering this kind of query for some reason; I literally can't think of five things I know.
(Note also that I have a general policy of keeping anything related to religion out of the rationality book - that there be no mention of it whatsoever.)