Although it has been "axiomatic" that our courts do not entertain suits to reform wills on the ground of mistake, appellate courts in California, New Jersey, and New York have decided cases within the last five years that may presage the abandonment of the ancient "no-reformation" rule. The new cases do not purport to make this fundamental doctrinal change, although the California Court of Appeal in Estate of Taff and the New Jersey Supreme Court in Engle v. Siegel did expressly disclaim a related rule, sometimes called the "plain meaning" rule. That rule, which hereafter we will call the "no-extrinsic-evidence rule," prescribes that courts not receive evidence about the testator's intent "apart from, in addition to, or in opposition to the legal effect of the language which is used by him in the will itself." The two courts said that they were consulting extrinsic evidence (primarily the testimony of the lawyers whose poor draftsmanship had led to the litigation) in order to engage in construction of supposedly ambiguous instruments. In truth, each of the two wills was utterly unambiguous. What each court actually did was to prefer the extrinsic evidence of the testator's intent over the contrary but mistaken language in the will.
Waggoner, Lawrence W. "Reformation of Wills on the Ground of Mistake: Change of Direction in American Law?" J. H. Langbein, co-author. U. Pa. L. Rev. 130 (1982): 521-90.