A will becomes invalid if the testator is shown to be mentally incompetent, according to Encyclopedia Britannica. A will is also invalid if the testator acted under fraud or coercion. About.com mentions that a will is void if all of the beneficiaries die without the testator appointing new heirs.
About.com adds that each state has certain requirements for validating a will. For example, a person must be 18 or over to draw a will. An understating of assets must be demonstrated, and the heirs must be named. At least two people must be present when the document is signed. If these state requirements are not met, the will is not recognized under the law, and an estate falls under intestacy laws. Wikipedia notes that intestacy laws determine who is eligible to receive the property or assets, such as a closest living relative. About.com reports that a previous will is recognized if a current will is rendered invalid.
If the testator remains alive, Encyclopedia Britannica reports that the testator must provide strict evidence that he was mentally unsound to draw up the will. A testator who claims delusion must also prove that his hallucinations determined the outcome of the will. Undue influence is another criteria for rendering a will invalid, and this includes being led to frame the document in a particular manner. Persuasion is not a reasonable ground to void a will, but overt threats are reason enough to rule in favor of the testator.