Leffingwell v. Warren was a case heard by the United States Supreme Court in 1862. The dispute arose from a contract between Leffingwell and Warren, where Leffingwell agreed to pay $500 for the right to use certain land owned by Warren for mining purposes. After paying half of the money, Leffingwell refused to pay any more due to his belief that he had not received what he was promised under the agreement. In response, Warren sued him for breach of contract and won at trial court level; however, on appeal before the US Supreme Court it was determined that there were material facts which could have been presented at trial but weren't which would have changed its outcome if they had been considered properly. As such, it reversed and remanded back down with instructions for further proceedings taking into account these new facts so as to ensure justice is done in this matter according to law.

In Leffingwell v. Warren, the Supreme Court was tasked with determining whether a deed of trust created by an individual in Massachusetts was valid under the laws of that state. The majority opinion held that it was not, as it did not meet certain requirements set forth by Massachusetts law. However, Justice Grier dissented from this decision and argued that although the deed may have been technically deficient according to Massachusetts law, its purpose should be taken into consideration when evaluating its validity. He reasoned that since both parties had intended for it to serve as security for a debt owed by one party to another and had acted accordingly in reliance on its terms, then those intentions should be respected even if they were not formally expressed in accordance with all legal requirements at the time of execution. Therefore he concluded that despite any technical deficiencies present in the document itself, it still served as sufficient evidence of an agreement between two parties which could be enforced through judicial action if necessary.