When I was in STL once, I saw people put peanut butter in chili.
Yes, some folks put it in as a "Secret Ingredient" probably to help thicken it --- but this needs to be disclosed now due to the prevalence of peanut allergies. It is also used in some enchilada sauces.
As a parent of a peanut allergic child, I've had to learn to ask about this very thing everywhere we go. We simply do not go to restaurants where nuts are regularly used in cooking like Thai and Indian cuisine. It is scary, though, when the risk is not so clear -- like in chili. Thanks for the reminder!