Contesting a will is very unusual. By one estimate, about 99% of wills sail through probate without a hitch. If a will doesn't fulfill certain legal requirements, or the maker of the will was not of age or sound mind, a would-be heir or beneficiary can challenge it in probate court after the will maker's death. There are several grounds on which someone who stands to benefit from getting the will thrown out can base a legal challenge.
It's almost never an issue, but the person who made the will must have been:
The maker of the will must have been of "sound mind" when the will was made. This is not a rigorous requirement. Usually, a court faced with resolving a question of mental capacity requires only that the person who made the will:
In reality, a person must have been pretty far gone before a court will rule a will invalid. Forgetfulness or even the inability to recognize friends doesn't, by itself, establish incapacity.
A will can also be declared invalid if a court determines that it was procured by fraud, forgery, or "undue influence." This usually involves some evil-doer who occupies a position of trust -- for example, a caregiver or adult child -- manipulating a vulnerable person to leave all, or most, of his property to the manipulator.
Learn more about undue influence.
What makes a document a valid will? For starters, every state has rules about what a will must, at a minimum, contain. Most states require that the document:
A typed or computer-printed will must have been dated and signed in the presence of at least two adult witnesses. In most states, the witnesses cannot be people who are named to inherit property under the will. (If a witness inherits, this may void the gift to himself but not the rest of the will.)
Handwritten, unwitnessed wills are valid in about half the states. These "holographic" wills must be written and signed entirely in the handwriting of the person making the will. (Some states, but not all, require that they be dated.) Because there are no witnesses, holographic wills are more easily challenged than standard typewritten wills; the probate court must be satisfied that the document is actually in the deceased person's handwriting and was intended to serve as a will.
Learn more about holographic wills.
Wills don't have to be notarized to be valid. Some wills, however, include a "self-proving" affidavit (sworn statement) that the witnesses sign in front of a notary public, which means the witnesses don't later have to come to court to swear that the will is valid.
Generally, a will is valid in any state where the maker of the will died if it was valid under the laws of the state (or country) where the maker of the will was "domiciled" when the will was made. A person's domicile is the state the person considered his or her permanent home -- where a person's principal home was and where the person spent the most time, as opposed, say, to a summer home.
To make your own valid will, use Nolo's Online Will or Nolo's Quicken WillMaker & Trust software. Both the online app and the software are customized to the laws in your state and provide detailed instructions for signing and storing your will.