Aflatoxin
Aflatoxins are naturally-occurring fungi-produced toxins that are found on foods like corn and peanuts and are known to cause liver cancer.  Developing countries are most at risk, but thankfully the FDA tests for these substances and has certain stated limits for ingestion.  This is of particular interest to me because of the frightening quantity of peanut butter I eat on a daily basis.  It's easy to find murmurs on internet message boards about how [X] food causes [Y] deadly problem, and this is no exception, but since there are so few (or perhaps no) documented cases of aflatoxicosis due to peanut butter ingestion, I'll continue eating it to my heart's content. #food

Radioactive bananas
Bananas are a good source of potassium, but some of that potassium comes in the form of the radioactive isotope potassium-40.  Thankfully the human body does a good job of self-regulating the amount of potassium it stores, so there isn't really a chance for a deadly buildup.  And just because bananas contain a radioactive substance doesn't mean they're unhealthy.  Different radioactive elements affect the body in different ways, so potassium-40 from bananas won't affect a person the same way as uranium-238 from a nuclear reactor.  Other foods with naturally-occurring radiation include Brazil nuts, carrots, potatoes, and lima beans. #food