Will challenges, particularly successful ones, are very rare. The legal grounds for contesting a will are limited to extreme circumstances. Basically, a will can be invalidated only if one were under age when it was made, mentally incompetent or if the will was procured by fraud, duress or undue influence.
Forgetfulness or even the inability to recognize friends does not by itself establish incapacity. The courts presume the will writer was of sound mind; a challenger must prove incapacity. Similarly, a will is rarely declared invalid on grounds it was procured by fraud, duress or undue influence; this requires proof some evil doer manipulated a person in a confused or weakened mental or emotional state to leave his/her property in a way the person otherwise would not have. If a will maker is improperly influenced a court is the right place to remedy the injustice.
If important circumstances change, your client may need to revise his/her will. In some states, if one gets divorced after making a will, the provisions that left property to the former spouse are automatically revoked. But that is not true in all states and it is not true for some kinds of property such as ERISA pension benefits. In any case, you should always revise your will and bring your estate plan up to date after a divorce. Never rely on state law for estate planning, especially for matters concerning an ex-spouse.
If your client has a child or gets married after the will is made, the will should be revised. If the client does not, the estate may become entangled in laws designed to protect spouses or children (as detailed earlier).
A will signed in one state remains valid if the client moves to another state. However, it is often advisable to draft a new will after a permanent move. The client may also want to appoint an executor who lives in the new state. Some states place extra rules and restrictions on out-of-state executors