Jeff Mullins is under investigation by New York racing officials after giving one of his horses a substance before a weekend stakes race . The California-based trainer said Monday it was an "honest mistake." Gato Go Win wasn't allowed to run in the $200,000 Bay Shore at Aqueduct on Saturday. New York Racing Association security officers said they saw Mullins giving a medication labeled as a cough remedy to the horse in the detention barn. Horses are kept there under secure conditions before a race. Mullins later saddled I Want Revenge to victory in the $750,000 Wood Memorial, making the colt a favorite for the Kentucky Derby on May 2. New York racing rules allow antibiotics, vitamins, electrolytes and other food supplements as long as they are administered orally and do not contain any other drug or act like a drug. But medication may not be given to a horse while it is in the detention barn, and that's why Mullins could face sanctions. The story was first repor