The discovery of warfarin is an example of Pasteur’s famous quote: “Chance favors the prepared mind.” During the 1920s, in the northern United States and Canada, the livelihood of ranchers declined when cattle started dying. Cows bled to death following minor procedures such as dehorning or castration, and even spontaneously. Herds were decimated. There was no identifiable pathogen or nutritional deficiency. The mystery drew the attention of a Canadian veterinary pathologist, Frank Schofield, who noted that all the affected cattle had eaten moldy silage from sweet clover plants. Sweet clover was a popular source of fodder since it grew in poor soil and was a great source of protein. In damp weather, the silage became infected with molds of Penicillium nigricans and P. jensi. Schofield separated good clover hay stalks from spoiled ones and fed them to two different rabbits. The rabbit that consumed the spoiled hay died. Schofield had discovered the anticoagulant properties of spoiled sweet clover hay. The hemorrhagic disease of cattle became known as “sweet clover disease” [1–5].