Professor Ian Ayres, from his Yale Coursera course American Contract Law, examines Sherwood v. Walker, the famous case of a cow named Rose 2nd of Aberlone sold as a barren breeding animal for beef value but discovered pregnant, and therefore far more valuable, after the sale. Ayres uses the dispute to teach the doctrine of mutual mistake as a formation defense to contract enforcement, distinguishing it from unilateral mistake and asking what kind of error about a bargained-for good lets a party escape a deal. He walks through the Michigan court's reasoning about whether the mistake concerned the substance of the transaction rather than a mere quality of the cow. The lecture is part of a unit on special controls on promissory liability within contract formation.