Which of the following circumstances can result in an invalid contract?

A contract is considered valid when it meets certain essential elements, one of which is that all parties involved have the capacity to contract. This means they must have the legal ability to enter into a binding agreement, which typically requires being of legal age and having sound mental ability.

When a contract is entered into by someone who lacks capacity, such as a minor, someone mentally incapacitated, or an intoxicated individual, the agreement may be deemed invalid. This is because the law recognizes that such individuals may not fully understand the implications of the contract or may not have the ability to give informed consent. Therefore, this circumstance directly affects the enforceability of the contract, leading to its invalidity.

The other options present scenarios that typically do not invalidate a contract. If both parties agree to the terms, the contract is generally valid. Signing in the presence of a notary adds a layer of authentication but does not affect the validity. Lastly, having detailed terms and conditions is generally beneficial and usually strengthens a contract rather than invalidates it. Thus, the circumstance of lacking the capacity to enter into a contract is what leads to an invalid contract.