'Frustration of purpose' as an excuse for nonperformance applies when:

Explanation

Frustration of purpose excuses performance when an event neither party foresaw destroys the principal purpose both understood as the basis of the bargain, even though the promised performance remains entirely possible — the textbook case is renting a room to watch a parade that is then cancelled. (b) is impossibility, where performance itself cannot be rendered; the doctrines are neighbours, not synonyms, and Civil Code §1511 sets out when performance is excused. (c) is commercial impracticability, which California applies narrowly: added cost, even large added cost, is the risk a fixed-price contractor accepted. (d) is buyer's remorse, which excuses nothing at all.