A contract or contractual provision may be unenforceable if it is unconscionable at the time of formation, meaning the terms are so unfairly one-sided as to shock the conscience. Unconscionability commonly arises where one party possesses significantly greater bargaining power than the other.

The defense of impracticability applies when an unforeseen event occurring after contract formation makes performance extremely difficult or unreasonably expensive for a party to complete.

Duress occurs when wrongful pressure or threats are used to coerce a person into entering a contract that the person otherwise would not have agreed to.

Impossibility discharges contractual duties when performance becomes objectively impossible due to unforeseen circumstances arising after contract formation.

Frustration of purpose discharges contractual duties when the contract's principal purpose has been destroyed. Performance is excused if: (1) the principal purpose of the contract is substantially frustrated, (2) the frustration is caused by an unforeseeable supervening event, and (3) both parties understood the principal purpose at the time the contract was formed.

A condition precedent makes a party's performance dependent upon the occurrence of a specified event. However, the condition may be excused if the protected party prevents or fails to cooperate in good faith with the occurrence of the condition, or voluntarily waives the condition.

If a written contract contains an incorrect price term that differs from the parties' prior agreement, the error may constitute a mistake in integration, allowing parol evidence to reform the contract. Additionally, if the agreement omits terms such as the time for performance or lacks a merger clause, courts are more likely to treat the contract as only partially integrated, permitting supplementary evidence consistent with the written terms.

Under the parol evidence rule, a party may not introduce evidence of prior or contemporaneous agreements that contradict the terms of a later integrated writing. Exceptions permit such evidence to show a condition precedent, correct a clerical error, establish a defense to contract formation, explain ambiguous terms, or supplement a partially integrated agreement.

The partial performance exception generally requires at least two of the following three elements: (1) full or partial payment, (2) possession of the property, and (3) substantial improvements made to the property.

Under Common Law, contract modifications generally must be supported by new consideration. Exceptions may apply when the modification is fair and equitable in light of unanticipated circumstances, involves a change in the parties' performance or promises, and the contract has not yet been fully performed. Under the UCC, however, contract modifications do not require consideration so long as they are made in good faith.

Exceptions to the UCC Statute of Frauds include: (1) a merchant's confirmatory memorandum that is not objected to within a reasonable time; (2) goods that have been received and accepted or paid for; (3) specially manufactured goods not suitable for sale to others; and (4) a judicial admission that a contract was formed.

Exceptions to the Statute of Frauds include full performance, judicial admission of the agreement, and promisory estoppel based on detrimental reliance. Additionally, a suretyship promise is also exempt and falls outside the Statute of Frauds when the promisor's main purpose in guaranteeing the debt is to secure a personal benefit.

Under the Statute of Frauds, certain contracts are unenforceable unless evidenced by a writing. These include contracts made in consideration of marriage, contracts incapable of being performed within one year, contracts for the sale of land or leases longer than one year, promises by an executor to personally pay estate debts, contracts for the sale of goods priced at $500 or more, and suretyship agreements. To satisfy the Statute of Frauds, the writing must be signed by the party to be charged, reasonably identify the subject matter, indicate that a contract exists, and include the essential terms of the agreement.

A breach is minor when the nonbreaching party still receives the substantial benefit of the bargain despite imperfect performance. A minor breach does not excuse the aggrieved party from continuing to perform under the contract, but it does entitle that party to recover damages caused by the breach.

A breach is material when a party fails to receive the substantial benefit of the bargain. In that event, the nonbreaching party is discharged from any further duty to perform and may immediately pursue all available remedies for total breach, including full contract damages.

A breach of contract occurs when a party fails to perform after all conditions precedent have been satisfied, the time for performance has arrived, and the duty to perform has not been discharged.

Satisfaction may be judged subjectively, so long as the party acts in good faith. Thus, even if a reasonable person would be satisfied objectively, the client's personal satisfaction controls if exercised honestly and in good faith.

A condition is an event that must occur before a party's performance becomes due. Concurrent conditions are conditions that occur at the same time, with each party's performance serving as a condition precedent to the other's performance.

A unilateral mistake occurs when only one party is mistaken about a basic assumption that materially affects the contract, and the other party is unaware of the mistake. Generally, a unilateral mistake is not a valid defense to contract formation. However, relief may be available if the nonmistaken party knew or had reason to know of the mistake, or had a duty to disclose the error.

A contract is voidable and may be rescinded or reformed when both parties are mistaken about a basic assumption underlying the agreement, the mistake has a material effect on the contract, and the party seeking relief did not bear the risk of the mistake.