I was reading something about this the other day. The article I read linked a common bacteria prevalent in rice that's kept in a rice cooker to keep warm to the headaches that people experienced. I don't know if I can find the article again, but if I can I'll post it.
EDIT: Here it is, under the section "Interview Highlights" I haven't checked this against other sources though.
In any case, I've only met one guy who was truly sensitive to the stuff and there was a long list of foods he had to be careful to avoid, much longer than the list posted in the article.