The most successful grounds for contesting a will are that the testator lacked the mental capacity to create a will or that he was unduly influenced to write the will a certain way. The most successful challengers are usually spouses of the testators, according to FindLaw.
Other reasons for contesting a will include a newer will trumping the one the executor presented for probate, insufficient or inappropriate witnesses, or that the provisions of the will are not valid for the state in which the testator resided at the time of death. As long as the will is legal, a contest is unlikely to succeed. Approximately 99% of wills pass through probate uncontested, notes FindLaw.