Raffles v. Wichelhaus is a landmark case in contract law that highlights the issue of mutual mistake regarding the subject matter of a contract. In this case, two parties entered into a contract for the sale of cotton to be delivered on a specific ship named the Peerless. However, both parties were referring to different ships with the same name, leading to a misunderstanding that voided the contract. This case is significant because it demonstrates how mutual mistakes can render agreements unenforceable when there is a lack of a shared understanding.