the occurrence of an unanticipated or extraordinary event may make contractual duties impossible or impracticable where neither party assumed the risk and the nonoccurrence was a basic assumption of the parties.
- must be objective
- must be after the contract has been entered into
acts as rescission where either party may sue for restitution. Party also has a possible quasi-contract
death or physical incapacity: does not relieve unless the person is necessary to effectuate the contract (ie unique)