In many states, a legal presumption of undue influence arises where a beneficiary under the will stands in a confidential relationship with the testator. For example, where a testator leaves property to the attorney who drew up the will.
A will may include an in terrorem clause, with language along the lines of "any person who contests this will shall forfeit his legacy", which operates to disinherit any person who challenges the validity of the will. However, since this clause is within the will itself, a successful challenge to the will renders the clause meaningless. Many states consider such clauses void as a matter of public policy.
Typically, standing to contest the validity of a will is limited to two classes of persons:
The following example is instructive: Monica makes a will leaving $5,000 each to her husband, Chandler, her brother, Ross, her neighbor, Joey, and her best friend, Rachel. Chandler tells Monica that he will divorce her if she does not disown Ross, which would humiliate her; later, Ross tells Monica (untruthfully) that Chandler is having an affair with Phoebe, which Monica believes. Distraught, Monica rewrites her will, disowning both Chandler and Ross. The attorney who drafts the will accidentally writes the gift to Rachel as $500 instead of $5,000; and also accidentally leaves Joey out entirely.
Under these facts, Chandler can contest the will as the product of fraud in the inducement, because if the will is invalid, he will inherit Monica's property, as the surviving spouse. Ross can contest the will as the product of Chandler’s undue influence, because Ross will inherit Monica's property if Chandler’s behavior disqualifies Chandler from inheriting (note, however, that many jurisdictions do not consider a threat of divorce to be undue influence). Rachel has standing to contest the will, because she is named in the document – but she will not be permitted to submit any evidence as to the mistake because it is not an ambiguous term. Instead, she will have to sue Monica's lawyer for legal malpractice to recover the difference. Finally, Joey is neither someone who stands to inherit from Monica, nor named in the will, and therefore is barred from contesting the will altogether.