Supervening impossibility is the impossibility arising after the formation of a contract. However, this arises at the time when the promisor's performance is due. Such impossibility usually arises due to facts that the promisor had no reason to anticipate and did not contribute to the occurrence of. If contracting parties were allowed to plead supervening impossibility, it would make the whole basis of contract insecure. Therefore, the risk involved in supervening impossibility could be deliberately excluded by stipulations in the contract.