The doctrine of 'impossibility' (or impracticability) may excuse performance when:

解析

Civil Code §1511 excuses performance prevented or delayed by an irresistible superhuman cause, by the operation of law, or by the other party, and the courts add commercial impracticability — extreme and unreasonable difficulty or expense that was not foreseeable, not merely more than was budgeted. The three distractors are one error in three costumes. (b) is estimating risk, which is precisely what a fixed price allocates to the contractor. (c) is reduced profit, which the doctrine has never covered. (a) is opportunity cost, not an excuse at all but a reason for a breach the contractor still pays for. Compare frustration of purpose, where performance stays entirely possible but the shared reason for it is gone.