Can a will be challenged because of insane delusions?
A will can be challenged on the basis that a person was suffering from insane delusions or mental derangement at the time they created the will. If one were successful in this will contest, the will would be deemed entirely invalid. However, showing that someone was suffering from insane delusions or derangement means more than showing that they had a misunderstanding or held false beliefs - it means that the person irrationally believed something and if they had been given evidence proving that their belief was false, they would have continued to believe it anyway.
In addition, the delusion or derangement must bear some connection to the will itself. For example, a will would probably not be invalidated just because the deceased believed that aliens were watching over him. However, if he believed that his son was one of these aliens and disinherited him because of this fact, a court would likely find this delusion sufficient to invalidate the will.