Within an hour of the planned feeding of a group of 300 Holstein cattle, a large number of cattle developed tremors, diarrhea, weakness, and paralysis. All cows exposed to the feed, a total of 159, died within 24 hours despite treatment attempts with atropine in approximately 100 animals. An additional 8 exposed animals were culled within a week. A thorough investigation demonstrated the accidental mixing of phorate into the total mixed ration instead of the intended mineral mix. Diagnostics confirming phorate exposure in deceased animals included brain cholinesterase determinations and the analysis of liver, rumen content, milk, and feed for phorate. Phorate is a restricted-use organophosphorus pesticide commonly used in US agriculture because of target pest efficacy, cost, and availability. Even with known human, animal, and ecological risks, organophosphorus pesticides remain the most widely used insecticides in the world today. This necessitates awareness about how to identify pesticide exposure in food animals and the corresponding public health risks for humans, including the potential for meat and milk residues and exposure in children. This report describes a dairy’s catastrophic loss of 167 cows caused by human error and assesses the public health implications therein. Veterinary diagnosticians, public health officials, and veterinarians must be prepared to collaborate in order to advise clients on case work-up, management, and preventive measures.