Sometimes a person who makes a will is mistaken about something important—how much property he owns, for example, or whether or not a close relative is still alive, or the effect of a certain will clause. After the person has died, unhappy family members may ask a probate court to change the terms of the will to reflect what they are sure was the will-maker's true intent.
Those relatives probably shouldn't bother hiring a lawyer. If a will is properly signed and witnessed, its language is clear, and the person who made it knew what the document said, courts almost never agree to hear evidence about the circumstances under which it was made, except for allegations that the will-maker was unduly influenced or lacked mental capacity. You can't blame the courts—if everyone were welcome to come in and argue about what Dad really wanted, the lawsuits would be endless.
For more about lawsuits challenging wills, see "Contesting a Will."
Generally, if you know and approve of the contents of your will, a court will not later throw out or change its terms, even if you included a provision in your will because you were mistaken about a fact. For example, take the case of a woman who left the "residue" of her estate—everything that's left over after specific gifts are paid—to a college, apparently thinking that the residue wouldn't be worth much. As it turned out, her assets were more valuable than she knew, and the college inherited more than half of her estate. After her death, there was a lawsuit, at which it became clear that the woman would have left more money to relatives if she had known how much she had actually owned. Still, said the court, it did not have the power to change the terms of her will "merely because it was the result of a mistake of fact on her part." (Holmes v. Campbell College, 125 P. 25 (Kan. 1912).)
In any case, it's not always clear, after a death, what was a "mistake of fact" and what was an opinion. For example, a Georgia man left little to his nephew after expressing the opinion that the nephew was "a ne'er-do-well who drank a lot." After the man's death, however, other relatives produced evidence that the nephew hadn't touched a drink in 20 years and worked until he retired. Was the uncle simply mistaken, and if so, should a court disregard the part of his will pertaining to the nephew? The court declined to do so. After all, it concluded, the deceased man lived less than a mile from his nephew, and they saw each other daily. If he made his bequest in the belief that his nephew was a lazy drinker, it was his deliberate decision. That his conclusion might have been factually wrong didn't give the court a reason to rewrite his will. (Yancey v. Hall, 458 S.E.2d 121 (Ga. 1995).)
Similarly, a man executed a codicil (amendment) to his will, revoking substantial cash bequests to two beneficiaries in the original will. In the codicil, he explained that he was revoking the will provisions because he had already given gifts to the two beneficiaries. In fact, he hadn't actually made the gifts. Too bad, said the court when the would-be beneficiaries challenged the codicil. The man knew whether or not he'd made the gifts, and he went ahead and signed the codicil; the court would not go back and change his words. (Witt v. Rosen, 765 S.W.2d 956 (Ark. 1989).)
Some states have made very limited exceptions to the general rule. For example, under Georgia law, if a parent makes a will while mistakenly believing his or her child is dead, the child may be entitled to inherit a share of the estate. (Ga. Code Ann. § 53-4-58.)
If, however, a will-maker makes a mistake by relying on someone's false statements, the result could be different. For example, the Vick family of Mississippi had seven children. Some of the daughters told their mother that she needed to leave all her property to the daughters, because their father planned to leave all of his property to the sons. The mother, relying on this erroneous statement, made a will that left her property to her daughters. (Estate of Vick, 557 So. 2d 760 (Miss. 1989).) Essentially, this is fraud—deceiving someone in order to profit. The Mississippi court characterized the daughters' acts as undue influence and set aside the mother's will.
If someone who's making a will is mistaken about facts because he or she isn't of sound mind ("lacks testamentary capacity," in legalese), a will is not valid. The difference between lack of capacity and being mistaken about a factual matter isn't always clear-cut. But generally, a mistake arises from external information; a delusion that would indicate lack of capacity arises from an internal delusion that has no basis in reason or reality.
Courts presume that someone who signs his or her will knows what the document says. If someone challenges a will and presents convincing evidence that the will-maker was mistaken about the contents of the will, the court could refuse to admit the sill to probate.
As a practical matter, if someone signs a will without knowing its contents, it's probably not just a simple mistake. More than likely, someone is exerting undue influence over a vulnerable person. For example, relatives of an 82-year-old man drew up a new will for him and brought it to him so sign shortly before his death, while he was hospitalized, weak and sometimes confused. After the man's death, his sons challenged the will, on the grounds that their father didn't know the terms of what he was signing and was unduly influenced. The court ended up ruling that part of the will was invalid, and noted that a will cannot express someone's "testamentary intent" if the person doesn't know what it says. (Estate of Turpin, 19 A.3d 801 (D.C. App. 2011).)
Courts sometimes, however, allow obvious mistakes in execution—that is, signing and witnessing—to be corrected. For example, a husband and wife signed each other's (identical) wills by mistake. After the husband died and the error was discovered, a New York probate court accepted the document the husband had actually signed, and switched the wife's and husband's names wherever they appeared in the will. (Matter of Snide, 52 N.Y.2d 193, 418 N.E.2d 656 (1981).)
A court is unlikely to accept a will for probate that had been signed, but not witnessed properly, even if the will-maker mistakenly thought it was valid. In a D.C. case, a will was challenged because only one witness had signed. (The statute required at least two.) Several people offered sworn statements stating that the deceased woman had intended the document to be her valid will and had told others that she had finalized her will. But whatever she may have mistakenly believed, the court ruled, she had not validly executed her will. (Estate of Henneghan, 45 A.3d 684 (D.C. App. 2012).)