A will may be held invalid if the will was procured as a result of a mistake. However, what does this mean? This doesn't necessarily refer to typos or minor changes which can be debated in probate court, these are major mistakes so big that the court will not allow the will into probate court. Courts have found that a mistaken belief by the testator, alone, will not invalidate a will in Estate of Carson 184 Cal. 437, 447-448. However, if the testator's mistake is a result of some type of fraud or undue influence these are separate causes of action and should be brought as grounds for a will contest.
Courts have invalidated a will for mistaken belief when the testator signed a document which he or she believed was something else, for example a sales contract but was in fact a will. This is a case of a mistake where the courts will invalidate a will because it was not the testator's intent to sign a will and having the intent to create a will is one of the basic requirements of wills drafting. If you are concerned with drafting your will or have a question about an existing will which is currently in Probate you should consult with a California Probate Attorney.