Writing a chord seems pretty simple – until you actually do it. One of the reasons why contracts drawn up by lawyers seem shrewd and superfluous is precisely because it is important to develop a language that could be applied in a decade by foreigners who have not been part of the negotiations and who have only the words on their side. What is “understood by the parties without saying so” cannot be understood in this way by a judge and jury who will interpret the agreement a decade after the death of a contracting party. In other words, most agreements are informal matters elaborated by laymen and the issue of vague wording, confusing wording or errors of a party regarding the object or intentions of the parties are common. If A and B entered into this contract with the understanding that the sale of marijuana is legal in the state of sale, both would operate under a reciprocal error of law and could both wind up the contract. In fact, the contract in Texas would not be applicable for reasons of public order. There can be no frequent error if the contract assigns the risk of the event that must be accidentally absent from the agreement. Normally, a unilateral error does not invalidate a contract.  Traditionally, it is Caveat Emptor (let the buyer be careful) and according to customary law Caveat Venditor (let the seller be careful). . . .