Often, the performance by parties to a contract varies from the written terms and conditions of the contract in slight, immaterial ways. Occasionally, the parties’ performance, either individually or collectively, may vary significantly from the contract’s express terms, giving rise to disputes and claims of breach of contract. Typically, contracts provide that their terms may not be modified or altered except by written agreement signed by all parties to the contract and the parties are precluded from offering evidence of other modification or amendment of the written contract. However, there are important exceptions to this rule.