This doesn’t happen very often, to be honest. When it does, it’s usually by someone who feels cheated out of his or her share of the will. However, legally, only a spouse is entitled to a specific share. So, to successfully challenge a will, it does little good to argue that the person making out the will made a mistake or cheated you. Instead, your best shot is to challenge the validity of the will, i.e. arguing the signature was forged, that the person making out the will was not of a sound mind, or that he or she was coerced or unduly persuaded by someone else.