Ian Ayres, teaching American Contract Law at Yale, uses Lenawee County Board of Health v. Messerly to explain the mutual mistake defense to contract formation. The case involves a house sold for use as a rental property that turns out to have a failed sewage system, a defect neither buyer nor seller knew about at the time of sale. Ayres walks through how courts decide whether such a shared mistake voids the contract, focusing on who bore the risk of the unknown defect and how the doctrine differs from unilateral mistake. The lecture sits within a broader unit on formation defenses and promissory liability, and treats the case as a vehicle for testing when courts will excuse a party from a bargain both sides entered under a false shared assumption.