A will contest is a formal objection to whether or not the will is valid. This is often based on the allegation that the will does not truly represent what the person who made the will wanted. In most cases, a will contest focuses on the person who wrote the original will being not of sound mind, or in legal terms, the person lacked testamentary capacity, was insane at the time the write was drafted, was pressured into signing a will (undue influence) or the will is a fraud. Will contests may tackle the whole will or just a portion of it.
In a large number of states, there is a legal presumption that undue influence is present when a beneficiary is in a confidential relationship with the testator, for example, if a person who dies leaves property to the lawyer who wrote the will. If a person does contest a will, it is up to them to prove undue influence.
Many wills do have a clause included in them, referred to as an in terrorem clause, that, in essence, states that those who contest the will lose their inheritance. This effectively disinherits the challenger. However, since this clause is a part of the will itself, if the will is challenged and the challenge is successful, the clause does not apply.