Can a will be challenged because of mistake?
A will can be challenged on the basis that a person prepared and executed that will because of a mistake about some law or fact. If one were successful in this will contest, the will would be deemed entirely invalid. The specific requirements for such a mistake vary from state to state, but generally, to successfully show a mistake you have to be able to show that someone had a mistaken belief about some fact or law, that their mistaken belief was a reasonable one for them to have, that they executed a will as a result of that mistaken belief, and that they would not have executed that will if they had not had that mistaken belief.
It should be noted that the key distinction between fraud and mistake is that with fraud, there is somebody else involved, whereas mistake is all about the person drafting the will just being mistaken about some fact or law.