When a testator, or will creator, chooses to make a will, he or she must generally sign and date a physical copy of the will in the presence of not one but two adult witnesses. However, even if the individuals who serve as witnesses are legally adults, they must meet some other requirements. If the individuals who serve as witnesses to a will fail to meet the requirements to do so, this may call the validity of the will into question and complicate the execution of the will when the time comes.
In addition to being legal adults, the individuals who serve as witnesses must be "uninterested parties," which means that they cannot generally benefit from the will they witness. Depending on the circumstances, this may be mildly flexible, if the benefit is tangential and the witness is not named as a beneficiary in the will.
If a witness is named as a beneficiary to a will, then a likely positive outcome is that a court may simply void the provisions of the will that outline the witness's gift or benefits. In less favorable outcomes, the will itself may fall under challenges from other parties who may claim that the witness in question influenced some aspect of the will. This can create enormous tensions between beneficiaries and other interested parties, often resulting in protracted conflicts that drain resources of the estate of testator.
If you have concerns about your relationship to a will that benefits you or believe that such will involves inappropriate witnesses, it is important to understand what the law actually has to say about the matter in detail. Many resources can help you examine your circumstances closely and protect your interests and rights from unfavorable outcomes.
Source: FindLaw, "Reasons to Challenge a Will," accessed March 30, 2018