Which contract is likely unenforceable under common law?

The reasoning behind selecting the contract formed by mistake as likely unenforceable under common law centers on the concept of mutual mistake. When both parties to a contract misunderstand a fundamental term of the agreement, it can render the contract void because there is no true meeting of the minds. A valid contract requires all parties to have a clear and mutual understanding of the terms involved; if there is a misunderstanding that affects the essence of the agreement, the contract cannot be enforced. This principle ensures that agreements are made with informed consent and are grounded in a mutual understanding of the obligations and rights arising from the agreement.

In contrast, the other options present scenarios that either meet legal requirements for enforceability or don't display the same critical fundamental flaw as a mutual mistake. A contract for the sale of goods worth over £500 is enforceable under the Sale of Goods Act, as it meets certain legislative requirements. A contract that fulfills the criteria for a formal agreement—such as being in writing and signed if necessary—would also be enforceable. Lastly, while verbal contracts can present issues in their enforcement due to lack of evidence, they can still be enforceable provided that there are no issues regarding consent or subject matter, unlike the critical issue of misunderstanding found in a mutual mistake scenario