In 1972, Franken Berry cereal, then a brand new product, was the subject of a paper in the medical journal Pediatrics because it caused a 12-year-old to have red poop. He had been hospitalized for several days for what appeared to be rectal bleeding but turned out to have been the red dye (FD&C Red No. 2 and No. 3) in the Franken Berry. In related news, Booberry can turn your stool green. "Benign Red Pigmentation of Stool Resulting from Food Coloring in a New Breakfast Cereal (The Franken Berry Stool)" (Pediatrics via Smithsonian)