As noted elsewhere in this book, "sanctity of contract" has been identified as one of the cornerstones of the classical model of contracts. However, as the previous chapter on mistake indicated, in certain limited situations parties may be excused from their contractual obligations. Frustration provides another example of an excuse from performance obligations. Whereas mistake deals with inaccurate assumptions or lack of knowledge about past or existing circumstances, frustration relates to inaccurate assumptions about future circumstances. Sometimes it is not clear whether a mistake or a frustration analysis is appropriate.