Yes, there are some circumstances in which a person's will could be invalidated. This normally happens when a person chooses to disinherit an heir, and the disgruntled heir files a complaint in probate court to contest the will.
Some of the most common bases on which will contest complaints are based are a lack of mental capacity, undue influence, fraud, and duress. If a court finds that a testator lacked the requisite mental capacity to execute a will, it will set the will aside. Also, if another heir or beneficiary fraudulently induced a testator to execute the will, exercised undue influence over a susceptible person to get him or her to execute the will, or committed a wrongful act in breach of a fiduciary duty, the will may similarly be set aside and that person could be liable for damages caused by their wrongful conduct. For more information on contesting wills or trusts, click HERE.