If the only mistake of the parties related to some matter, either of fact or law in the situation of affairs prior to the execution of the writing in question, making it desirable or undesirable to enter into a written contract or conveyance, there can generally be no question of reformation. Occasionally the object of the parties has been so clearly agreed upon between them that equity by reforming a writing can give effect to that object, though because of their erroneous belief as to their antecedent rights, they sought to attain it by inadequate means; but usually the only question can be of rescission, and in determining the propriety of such relief, it must first be asked whether the mistake related to an essential matter assumed by both parties as a basis of their agreement. Only after that question has been answered in the affirmative can it make any difference whether the mistake is of fact or of law. It is undoubtedly generally said in broad terms that if the mistake is of law, no relief is possible. Thus it has been said, distinguishing mistake of law of this sort from mistake of law in the expression of an agreement, where, as already observed,44 relief is generally allowed:45 "If ... in a given case the parties actually mistake or misunderstand the principle of law applicable to the subject-matter of the contract and reach an agreement relying upon this mistake of the law, there is no ground upon which a court of equity can reform the contract. . . .46 When, however, the mistake lies not in a misunderstanding of the principles of the law as controlling the subject of the contract or the rights of the parties connected therewith, but merely in the terms proper to be used in defining the actual contract of the parties, such a mistake, though in one sense a mistake of law, is one that a court of equity will correct."47
44 Supra, Sec. 1586.
45 Abraham v. North German Ins. Co., 40 Fed. 717.
46 It is not clear, however, that there might not be rescission. But see Griffith v. Sebastian County, 49 Ark. 24, 3 S. W. 886.
A bond executed in conformity with a statute thereafter held unconstitutional is executed under a mistake of law, though it has not always been appreciated that in so far as the instrument would be a valid contract at common law, any relief from it must be based on mistake of law. Whether such a bond is void, it is held, depends upon the consideration of the bond and public policy. If there is no infringement of policy and the obligor gave the bond under no sense of compulsion and received consideration, the contract is upheld generally on the ground of a so-called estoppel.48